President Reagan capitulated to wildlife and conservation groups yesterday, naming Robert A. Jantzen director of the U.S. Fish and Wildlife Service.

The nomination ends a three-month-long guessing game over whether the job would go to Jantzen or to Norman C. Roberts, a Californian and longtime Reagan supporter whose candidacy had been opposed by major conservation groups.

Ten major conservation organizations wrote to Reagan in July opposing Roberts, saying the financial analyst and former veterinarian did not meet the job's statutory requirements, which state that the director must have education and experience in the field.

Jantzen, a former director of the Arizona Fish and Game Department, had been selected for the job as Fish and Wildlife Service director by Interior Secretary James G. Watt, and was already in Washington working as a special assistant to Watt when Roberts' name popped up in early July as the White House choice for the post.

Although Watt repeatedly maintained that Jantzen was his man, Roberts soon joined the Interior Department as an aide to G. Ray Arnett, assistant secretary for fish, wildlife and parks.

The plot thickened in mid-July, when Roberts, speaking at a get-acquainted session arranged by the White House, told Washington wildlife professionals that the highlights of his career included being graduated in the top four-fifths of his class and being fired for attempting to remove the ovaries from a male cat.

Roberts later described the comments as "tongue in cheek," but his audience was appalled. The conservation officials also were taken aback by Roberts' statements that, as director, he intended to follow orders rather than make policy, and they quickly mounted a campaign against him at the White House and on Capitol Hill, where his nomination would have to be confirmed by the Senate.

Conservation officials said yesterday that, because of the statutory requirements, Roberts would have faced stiff opposition in the Senate. "The Senate is generally inclined to let the president make his own mistakes," said one official. "But in this case members were inclined to be less than charitable." Jantzen is expected to win easy approval.

Roberts is still listed as a special assistant to Arnett, but is on leave without pay and has been back in California since "sometime in August," according to the Fish and Wildlife Service.

The National Wildlife Federation, one of the groups that led the charge against Roberts, hailed the nomination of Jantzen yesterday as a "victory for professionalism," and said it was gratified that Reagan had "taken the advice of the nation's major conservation groups."

Meanwhile, a spokesman characterized the mood at the Fish and Wildlife Service as "elated." The office has been leaderless since the inauguration, said spokesman Alan Levitt, and "this is the best news we've heard in 10 months."