The president of a Beltsville sewer and water construction company pleaded guilty yesterday to giving false information on an income tax return and was sentenced to a year in jail with all but 30 days suspended.

Frank T. Hauser, 38, president of Tri-County Utilities Inc., was sentenced by Prince George's County Circuit Judge Robert J. Woods, who also ordered him to perform 200 hours of community service.

Prosecutor Joseph L. Evans said Hauser had underreported his personal income by $50,000 on his 1978 and 1980 personal returns, and by $100,000 on corporate returns for 1979, 1980 and 1981.

The guilty plea is the first to result from a continuing investigation begun by state prosecutors 20 months ago after a series of articles in The Washington Post. Those stories detailed allegations of bid-rigging and price-fixing among sewer and water construction companies competing for contracts awarded by the Washington Suburban Sanitary Commission, the public agency that builds and maintains sewer and water lines in Montgomery and Prince George's counties.

The charge to which Hauser pleaded guilty is a misdemeanor. Six other charges against him, including obstruction of justice, were dropped.

Tri-County Utilities vice president Lee Ganey Jr. is charged with seven counts of income tax evasion.

Marvin F. Bosley and Otha D. Upchurch, owners and officers of Suburban Utilites Contractors Inc., another Beltsville sewer and water construction company, have also been charged with tax violations in the investigation.