Specimens of German cockroaches (Blattella germanica) from two different locations near Owensboro, Ky., USA, were tested in the laboratory for resistance to diazinon and malathion. Those from one location were reared for three generations, and those from the other for four generations, before testing was completed. The results showed that both strains possessed resistance to diazinon, but neither strain was resistant to malathion. The males and females of one strain were, respectively, 2.5 and 2.6 times resistant to diazinon at LC(50) and 3.0 and 3.8 times resistant at LC(90): while those of the other strain were 5.8 and 8.0 times resistant to diazinon at LC(50) and 9.3 and 15.3 times resistant at LC(90).