The toxicity of fatty acid salts to German, Blattella germanica (L.), and American cockroaches, Periplaneta americana (L.), was evaluated. Potassium and sodium laurate caused up to 95% mortality of German cockroaches and 100% mortality of American cockroaches. Even-numbered potassium fatty acid salts, C₈–C_18, were assessed for toxicity at 0.125, 0.25, 0.5, 1, and 2% concentrations by a 30-s immersion of cockroaches. The more soluble of the fatty acid salts at 2% concentration caused 65–95% mortality of German cockroaches and 100% mortality of American cockroaches. Potassium oleate, C₁₈, was most toxic to both German (LC₅₀ = 0.36%) and American (LC₅₀ = 0.17%) cockroaches. Fatty acid salt solutions on a substrate were tested by placing cockroaches in contact with treated floor tiles immediately after application (wet) or after the solutions had dried. Sodium laurate and potassium caprate caused mortality of German (62 ± 17.4 and 58 ± 12.6%, respectively) and American cockroaches (52 ± 18.5 and 28 ± 4.9%, respectively) on wet tiles, whereas potassium oleate caused mortality of German cockroaches (67 ± 14.1%) only. Dry fatty acids caused no mortality among exposed cockroaches. Fatty acid salt solutions can be effective in killing German and American cockroaches but only when insects are thoroughly wetted with 1–2% fatty acid salt solutions.