The rate of increase and rate of regression of resistance was followed in Diazinon® (O,O-diethyl O-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate)-resistant house flies (Musca domestica L.) which show cross resistance to a wide variety of insecticides. The parent strain was collected in the field and selected each generation with Diazinon; increase in resistance was measured in this strain. Continued selection with Diazinon resulted in maximum resistance levels of 10-fold to malathion, Isolan® (1-isopropyl-3-methyl-5-pyrazolyl dimethylcarbamate) and Dimetilan® (2-dimethylcarbamyl-3-roethyl-pyrazolyl-(5)-dimethylcarbamate), 5-fold to dimethoate and Dibroru® (1,2-dibromo-2,2-dichloroethyl dimethyl phosphate), 40-fold to ronnel, 700-fold to lindane, and 125-fold to Diazinon. These, however, may be temporary plateaus. Regression of resistance occurred gradually for all the materials mentioned above and for DDT. After 35 generations of no-selection, however, the strain had reverted to normal susceptibility for one material only, dimethoate. These studies showed that the decreased susceptibility to pyrethrins and Lethane 384® (beta-butoxy-beta-thio-cyanodiethyl ether) detected in the strain when collected from the field was probably vigor tolerance rather than true resistance.