A new macrolide, micromonospolide A, was isolated from an undescribed actinomycete, Micromonospora sp., and its structure was elucidated to be a bafilomycin-type macrolide which has a 16-membered lactone ring on the basis of spectroscopic data. Micromonospolide A inhibited gastrulation of starfish embryos at a concentration of 10 ng/ml or greater.