This study investigated the occurrence of slime material in the Sacramento River downstream of the Sacramento City Main Treatment Plant. Results of a six-month field program indicate that the slime condition consists of filamentous bacteria, primarily Sphaerotilus-type organisms. The problem affects a ten-mile reach of river for a two-month period during the summer and has aesthetic and nuisance effects on fishermen and marina owners. Information regarding the metabolism of the slim organisms, together with an analysis of the treatment plant effluent and receiving water conditions, indicate that filamentous slime growth is primarily caused by carbon compounds present in the treatment plant effluent. Other factors such as river velocity and bottom conditions and chlorine levels influence the extent of growth. It is expected that the magnitude of slime growth will be considerably reduced when upgraded regional wastewater treatment facilities are completed.