Bioactivity-guided fractionation of Streptomyces microflavus neau3 fermentation broth was used to determine the chemical identity of bioactive constituents. The structure of the new compound 1 was determined on the basis of spectroscopic analysis, including 1D and 2D NMR, as well as HRESI-MS, ESI-MS, UV and IR, and compared with the data reported in the literature. The acaricidal and nematocidal activities of the isolated compound 1 and its previously reported analogs 2, 3, 4 and 5 were evaluated. The results demonstrate that compounds 1, 2, 3, 4 and 5, especially compound 1, have potential as antiparasitics with acaricidal and nematocidal activities. These lactones shed new insights into the structural diversity of nemadectin and milbemycin analog libraries that are potentially accessible by combinatorial biosynthesis.