Twenty-two strains of soil bacteria, including representatives of the genera Bacillus, Micromonospora, Mycobacterium, Nocardia, Streptomyces, Thermoactinomyces, and Pseudomonas and 10 unidentified gram-negative, motile, rod-shaped bacteria, were shown to degrade aldrin to its epoxide dieldrin. In every case, the exo-stereoisomer of dieldrin was produced exclusively.