When you are the worst team in the National Basketball Association, you can't afford to practice curtain calls.

Nevertheless, the Houston Rockets, who have won only six of 38 games this season, ran a perfect play in the final seconds tonight to beat the Bullets, 100-98, and extend Washington's losing streak to six games.

With four seconds left and the game tied at 98, Houston's Wally Walker looked for Caldwell Jones and Allen Leavell. He saw they were covered, and passed to Joe Bryant, who drove past Carlos Terry to score the winning basket with one second left.

The victory left Houston Coach Del Harris smiling in his unaccustomed role as winner.

"It's the president on the phone!" someone called out as Harris answered questions after the game.

Harris almost started for the phone.

For the Bullets, it was a night in which guard Frank Johnson returned to the roster for the first time since Dec. 20, when he suffered a chip fracture of his right elbow. Johnson, looking as quick as ever, scored 14 points and had six assists in 31 minutes.

To make room for Johnson, the Bullets put Kevin Porter on waivers.

"There wasn't any problem with pain except when I fell once," said Johnson. "I've just got to get my wind back and my shooting going."

The fourth period began with the Bullets trailing, 79-78. They led, 54-50, at the half. At the start of the final period, it appeared as if the big, old men of the Rockets--Elvin Hayes, Jones and Billy Paultz--would put the game out of reach. They shot well from the floor while Washington faltered.

With 6:55 left and Houston leading, 90-84, Jeff Ruland went to work, and he was practically alone in his efforts.

Ruland scored 13 points and had seven rebounds in the period--he finished with 22 points--and tied the game at 96 on a three-point play with 1:17 left.

After both teams failed to score, the Bullets called time with 29 seconds left. Washington Coach Gene Shue substituted John Lucas for Johnson, but Lucas threw the ball away to Bryant. Lucas then fouled Leavell, whose two free throws put the Rockets ahead, 98-96, with eight seconds left.

After a timeout, Ruland took a pass from Rick Mahorn and scored on a layup to tie the game. With four seconds left, the Rockets called time and planned the play that would give them their sixth win of the season.

"I was the last option," said Bryant. "As soon as I caught the ball with four seconds left, I went for the basket."

"On that play, we made a very bad mistake," said Shue. "We shouldn't have had single coverage on Bryant's drive . . . It was also turnovers that killed us. We turned it over 27 times to their 15. And that's not to a very aggressive team."

Of his scoring efforts, a disgusted Ruland said, "I guess it just wasn't enough."

Hayes, a 15-year veteran of the NBA and ex-Bullet, may break Wilt Chamberlain's record for most minutes played in a career when the Rockets play at Denver Wednesday night. He needs 40 more minutes.

Hayes trails Chamberlain and Bill Russell on the all-time rebounding list; he is fifth in career scoring.

After the game, Hayes talked about the Rockets' difficulties since the departure of Moses Malone for Philadelphia.

"I'll tell you. It's much better playing on a team like this than on the team next door," he said, pointing to the Bullets' dressing room. "There's no pressure on us to make the playoffs. Maybe we can draft Ralph Sampson or somebody and build for the future."