Insect emanations occasionally cause allergic asthma, which not infrequently is due to occupational exposures. This second report of asthma caused by sewer flies, Psychoda alternata, concerns a sewage plant worker who previously had developed immediate-type hypersensitivity to wax moths. Evidence for sewer fly allergy was derived from direct prick and intracutaneous skin tests, Prausnitz-Kustner testing, in vitro leukocyte histamine release, ELISA, and bronchial provocative challenge. The ELISA inhibition tests indicated little cross-reactivity between sewer fly and wax moth extract or midge hemoglobin, but direct skin testing suggested possible hypersensitivity to other families of the order Diptera.