A pyrethrins-susceptible and a pyrethrins-resistant strain of German cockroaches, Blattella germanica (L.), were selected with either permethrin or fenvalerate over several generations to determine if resistance would occur. Large nymphs were left on treated glass surfaces until about half were killed each generation. The level of resistance was determined periodically by time-mortality testing. The VPI-susceptible strain served as the basis for comparison. The pyrethrins-susceptible strain developed resistance to pyrethrins in six to seven generations with either selecting agent. This strain also showed resistance to allethrin when fenvalerate was the selecting agent. The pyrethrins-resistant strain developed resistance to allethrin with both selecting agents; in addition, resistance to permethrin, phenothrin, and fenvalerate developed when fenvalerate was used. The synergists piperonyl butoxide and MGK 264 (2-(2-ethylhexyl)-3a,4,7,7a-tetrahydro-4,7-methano-1H-isoindolel, e(2H)-dione) largely negated resistance to pyrethrins, but neither they nor the esterase inhibitor DEF (S, S, S tributyl phosphorotrithioate) were effective after development of resistance to allethrin. While resistance to pyrethroids can easily develop in this species, both the selecting agent and the previous history of resistance appear to have a bearing on how resistance develops.