A pilot horizontal biological filter similar to a trickling filter was constructed on Jackson Creek near Lake Delavan, Wisconsin. The filter consisted of graded rocks in three sections of four feet wide by two feet deep by six feet long. Water was pumped from Jackson Creek into the filter and water chemistry and bacteriological analyses were undertaken on the influent and effluent from the filter. There was little effect of the filter on the water chemistry or bacteriological content. Preliminary results indicate that substantial B.O.D. reduction occurred when the influent was well oxygenated.