Adult male Woodhouse's toads (Bufo woodhousi) developed clinical disease, hepatomegaly, and died at a higher rate when externally exposed once to either a high or low sublethal dose (0.011 or 0.0011 mg malathion/g toad) of field grade malathion and challenged with a sublethal dose of Aeromonas hydrophila injected intraperintoneally (1.1 x 10(4) bacteria/g toad) when compared to toads not exposed to malathion but challenged with A. hydrophila (P < 0.007). Toads exposed to malathion (high or low dose) and challenged with A. hyydrophila had clinical disease, hepatomegaly, and died at a higher rate [9 (90%) of 10] than toads exposed to malathion alone (P < 0.002). Toads exposed to the high and low doses of malathion had a 22% and 17% decrease in brain cholinesterase levels, respectively, when they were compared to nonmalathion exposed toads (P < 0.025, P < 0.006). It appears that field grade malathion applied externally to adult Woodhouse's toads may cause increased disease susceptibility when challenged with a potentially pathogenic bacteria.