Plexus Resources Corp. has announced a net loss of $402,000, 3 cents a share, for the quarter ending Dec. 31, down from a loss of $261,000, 5 cents a share, in the same period of 1989.

The company's loss for the six months ending Dec. 31 was $819,000, 7 cents a share, compared with a loss of $522,000, 10 cents a share, in the period a year earlier.The Salt Lake-based company has changed its year-end from June 30 to Dec. 31. A spokesman said the increased losses were due partly to low sales margins during start-up operations at the company's 24 percent-owned Denton-Rawhide mine.