The dispersibility of three liquid formulations of Bacillus thuringiensis serotype H-14 was compared in field tests against Psorophora columbiae larvae. The materials (1.89 liters diluted to 20.8 liters) were dispensed at 80 ml/min irrigation water entering rice fields. The area covered was assessed 48 hours post-treatment by larval sampling of the periphery of the treated pan and all downfield pans flooded at that time. Area affected by dispersed B. thuringiensis (H-14) was determined by delineation of the counts of zero to 1.0 larva/dip with many zero counts versus areas of 1.1 or more larva/dip and a few samples of zero larvae. No significant differences occurred in area of dispersal or in the percentage of the total flooded area into which they were dispersed. The study also resulted in establishment of a new method of testing biological control agents applied to rice field irrigation water.