A new larvicidal antibiotic, aculeximycin, was found in the culture broth of an actinomycete identified as Streptosporangium albidum. Aculeximycin was isolated from the culture filtrate by adsorption on a Diaion HP-20 column and successive elution with acidic aqueous acetone. It was extracted from the concentrated active fraction with 1-butanol and subjected to column chromatography on a Sephadex LH-20 column. Aculeximycin exhibited strong larvicidal activity against mosquito larvae as well as antimicrobial activities against Gram-positive and Gram-negative bacteria, yeasts and molds.