The defending world champion lost to a relative unknown in the first round of the world computer chess championship here, and suddenly Canada was a front-runner in what had been shaping up as a contest between the United States and the Soviet Union.

Chess 4.9, one of the best-known chess players in the world although it is only a computer program developed by a team of experts at Northwestern University, lost to a small but fast-thinking program called L'Excentrique, designed by Claude Jarry of Montreal, playing through a long-distance telephone line on a computer at McGill University.

The first-round loss stunned spectators, many of whom have been watching Chess 4.9 career since 1970, when it was named Chess 3.0 and was not nearly as good at the game as it is now, but was the strongest chess program in the world. (The number in its name is changed one decimal point each time the program is improved significantly). Chess 4.9 will still be world champion until Monday, when the tournament ends and the title is awarded to another of the 18 competing programs from America, Russia, Canada, England, Sweden, Germany and Czechoslovakia.

In September 1970, when computer chess was still a strange idea to most people, a team from Northwestern entered Chess 3.0 in the first U.S. computer championship and it won every game it played. It remained undefeated through the next two U.S. championships, while its name climbed to Chess 3.5 and it began to gain a reputation as a sort of electronic Bobby Fischer.

But how new programs were being developed. Chess 4.0 won the fourth U.S. championship in Atlanta in 1973, but for the first time another program held it to a draw. Then in 1974 the first world championship was held in Stockholm. Chess 4.0 lost to a fellow American program named Chaos. And along came a dark horse named Kaissa, from the Institute for Systems Studies in Moscow, that swept to the world championship without losing a single game. (Curiously, for an atheistic program, Kaissa is named after the goddess of chess -- who was invented, or discovered or whatever you do to with goddesses, during the Renaissance, despite the ancient Greek sound of her name.)

A few months after the world championship, Chess 4.0 lost the U.S. championship for the first time. But unlike some humans, chess-playing computers never quit when they are losing, even when the odds are overwhelming. Nursing its wounds, Chess 4.0 continued to build character. It now plays consistently near master level and has even won a few games from human masters.

In 1978, it was the program chosen to play against British master David Levy in a climactic confrontation between man and machine in Toronto. It lost, but it lost gamely, and it drew blood, scoring a win and a draw before the match was over. It was also in Toronto, in 1977, that it had become world champion, ending the second world championship tournament with a perfect score, Kalissa, the following champion, lost in the first round to an American program called Duchess (from Duke University -- get it?). Now, for the second tournament in a row, the world champion has been knocked out in the first round. Perhaps a tradition is being established.

After the first world championship, one of the contestants remarked that to play, "a contestant merely has to arrive on time." It isn't like that anymore. The 18 programs competing here had to earn that right through previous tournament results. L'Excentrique, the first-round spoiler, was not even eligible until it won a match against two other Canadian programs in August. The program's first-round success was as much a surprise to its designer, Claude Jarry, as to anyone else. "I came hoping to finish among the first six," he said after the game. "Now, maybe I can do better."

Chess computers as a whole are doing better than ever before, as is clear at this world championship. The tournament is being held in the Brucknerhaus, an ultra-modern concert hall and center for international meetings located in a park overlooking the Danube. Outside the concert hall where the games are being played, the corridors are lined with booths showing commercial game-playing machines. When the spectators get tired of watching the world-class computers, they can wander out to a manufacturer's exhibit and play against an electronic opponent -- not only chess in a variety of packages but also checkers, backgammon and even bridge.

In terms of noise, at least, the scene is dominated by the VSC, the Voice Sensory Challenger from Fidelity Electronics of Miami. There is a German edition on display that announces each move as it is made in the dead-pan monotone associated with robot speech. "Von F3 ZU G5, springer nehmt laufer." ("Knight on KB3 takes bishop on KN5.") Up a short flight of stairs, but audible when the VSC is not chattering, is a German program, Schachmeister, that displays the game on a colored television screen and plays a little melody each time a move is made -- a different tune for each kind of piece in the game.

Saragon, the program that beat a $5 million computer in the 1978 North American Championship using a $5,000 computer, is now commercially available in a variety of packages, including a "pocket" set that might fit in a topcoat pocket but would have trouble getting into a tight pair of jeans. The most elaborate format is a deluxe "auto-response board" costing more than $2,000; it knows what you are doing without being told. Most chess computers have a keyboard, something like an adding machine, which is used by the human player to enter his moves. But the "auto-response board" has sensors under each square that tell it which piece has been moved and where. It signals its own moves by lighting up the squares where the move begins and ends.

Chess Challenger has a smaller, less expensive board that operates the same way, does not have Sargon's air of luxury, but is a tough competitor. A slightly more advanced model of the same process is competing in the world championship -- "about $200 worth of electronics against million-dollar hardware," as one member of the Fidelity Electronics team said prouldy.

The $200 machine just lost its first-round game to Duchess, which is by now a well established first-round spoiler -- but this was hardly a surprise. Outside of the defeat of Chess 4.9, the first round went approximately as expect, with victories scored by Kaissa (from Russia), Master (from England) and Belle, Chaos and Duchess, all from the U.S. With Chess 4.9 out of serious competition, Americans hopes here are now centered on Belle, a program developed by Bell Laboratories.