The maximum parasitism rate byDoryphorophaga doryphorae(Riley) of the Colorado potato beetle (CPB),Leptinotarsa decemlineata(Say), approached 75%. However, this occurred in late August and September, when the CPB larvae had already damaged the crop. The effectiveness of the parasite was limited by its low abundance during the first generation of the CPB. Although 9, 30, and 61% of the 2nd, 3rd, and 4th larval instars of CPB, respectively, were parasitized, parasitic maggots did not develop until the fourth-stage CPB larva entered the soil to pupate. Thus, there was a delay of about 7 days in fly emergence from hosts that had been parasitized as 2nd-instar rather than 4th-instar CPB larvae. We estimated that as much as 39% of the emerging fly were delayed over a 7-day period