Field tests were conducted to determine the relative toxicity of Dylox® (O, O-dimethyl 2,2,2-trichloro-l-hydroxyethylphosphonate) and other insecticides to certain entomophagous insects which aid in regulating the population density of field crop pests in California. Dylox was applied on alfalfa hay at dosage rates of 3.6 to 11.4 ounces per acre. At higher dosages, Dylox was quite toxic to larvae of the convergent lady beetle (Hippodamia comergens G.-M.).H. parenthesis (Say), and H. quinquesignata punctulata LeConte, but only moderately toxic to the adults. Nabis ferus (Linn.), Orius sp., and one of the Aphidiine parasites, Praon palitans Muesebeck, of the spotted alfalfa aphid, Therioaphis Maculate (Buckton), also were quite susceptible to higher dosages. At dosage rates of approximately one-half pound or less, Dylox was only slightly to moderately toxic to coccinellid adults and larvae (Hippodamia spp. mentioned above),Chrysopa spp., and Geocoris spp. Parathion, which was used as a standard comparison material, was highly toxic to the entomophagous species at dosage rates of 3.6 to 8 ounces per acre. A DDT-toxaphene mixture (1.3 and.2.6 pounds, respectively), used to control many field crop pests in California, proved to be extremely toxic to H. convergens, Geocoris spp., Orins sp., Cltrysopa spp.,Nabis ferus, and Sinea diadema (Fabr.), and to syrphids. DDT applied alone (1.3 pounds per acre) was nearly as toxic to the entomophagous species as the DDT-toxaphene combination. On the other hand, toxaphene (2.7 pounds per acre) was not nearly so toxic as DDT and was far less toxic than the DDT-toxaphene mixture. Sevin® (N-methyl-1-naphthyl carbamate), applied at 19 ounces per acre, caused marked reductions in H. convergens, adults and nymphs of N. ferus, and adults of Collops vittaus (Say). Heptachlor applied at 2.4 ounces per acre was moderately toxic to adults of Geocoris spp, and C.vitlatus, whereas at this dosage heptachlor had essentially no effect on H. convergens, and adults and immatures of Orius sp., Chrysopa spp., and N. fetus.