The mutagenic effects of diesel emissions was studied in the fruit fly Drosophila-melanogaster. Approximately two hundred 2 to 5 day old wild-type male fruit flies were exposed to an air stream consisting of diesel engine exhaust gases diluted fivefold with filtered ambient air, and a sex linked recessive lethal test was performed. Randomly selected 100 exposed and 100 control flies were individually mated to two 3 to 5 day old virgin females on the second day postexposure, and again mated to fresh virgins on the eighth day postexposure. Seven hundred F1 females were mated to two brothers. The resultant F2 cultures were scored for a sex linked recessive lethal event. F3 cultures were scored in the same manner. The diesel exhaust mixture was analyzed and contained: carbon- dioxide (124389) 0.17 percent, carbon-monoxide (630080) 12.2 parts per million (ppm), hydrocarbons 11.6ppm, nitrogen-oxide (10102439) 3.8ppm, nitrogen-dioxide (10102440) 2.9ppm, sulfur-dioxide (7446095) 1.0ppm, and particulates 2.18mg/m3. The number of recessive lethals found in the F2 and F3 treatment groups of both broods was smaller than in the control group (F2, 0.30 percent; F3, 0; 0.37 percent and When F2 and F3 totals were combined, the control group showed a rate of 0.29 percent versus 0.20 percent for the exposed group.