Insecticide resistance in the German cockroach, Blattella germanica (L.), is a significant challenge to the pest management professionals worldwide. We collected 24 field populations of B. germanica from different localities in Taiwan island, reared them for one to two generations, and evaluated them for their resistance to deltamethrin, propoxur, and fipronil using the surface-contact method. Results showed that deltamethrin resistance ratio ranged from 1.5 to 817.5×. Among the strains, TC Supermarket, TC Sanshang Logistics, TCTHSR, and TC 1Taichungsteak strains showed very high resistance to deltamethrin, which mortality ranged between 0 and 33% at 7-d post-treatment. On the other hand, resistance to propoxur and fipronil RR were 0.70–7.13× and 1.67–3.72×, respectively. Synergism studies using piperonyl butoxide (PBO) and S,S,S-tributylphosphorotrithioate (DEF) suggested the major involvement of cytochrome P450 monooxygenase and minor involvement of esterases. However, deltamethrin resistance in two strains (i.e., TC Supermarket and TCTHSR) was not affected by both PBO and DEF, indicating that other mechanisms are involved in the resistance, including kdr resistance. Evaluation of the field strains using commercial gel baits containing fipronil, imidacloprid, hydramethylnon, and indoxacarb for up to 7 d resulted in 24.4–100%, 11.3–78.5%, 15.8–75.5%, and 63.3–100% mortality, respectively. We found that high deltamethrin resistance in some strains could affect the performance of fipronil, imidacloprid, and indoxacarb baits, indicating the potential involvement of cytochrome P450 monooxygenase in reducing the effectiveness of the bait toxicants.