The possible presence of protein δ-endotoxins with homology to a cloned mosquitocidal Bacillus thuringiensis var. israelensisδ-endotoxin was investigated in two other mosquitocidal B. thuringiensis strains. Antiserum raised against the var. israelensis 27-kDa toxic polypeptide cross-reacted strongly with a 27-kDa crystal polypeptide of var. PG14 (serotype 8a:8b, a serotype more usually associated only with lepidopteran toxicity). In addition hybridisation studies using a DNA probe derived from the cloned var. israelensis 27-kDa δ-endotoxin gene located a homologous region on a 72–74-MDa plasmid of PG14. A less toxic strain, var. Kyushuensis, showed neither immunological cross-reaction with the antiserum nor DNA hybridization with the probe.