A microsporidium infecting the predacious mosquito Culex fuscanus Wiedemann, collected from Liu-Chiu Islet of Taiwan, was shown to be heterosporous. Two different types of haploid spores, one oval and the other lanceolate, were concurrently produced in the infected larvae. Merogony preceding the sequence leading to oval meiospores ended with the formation of a binucleate sporont with similar ultrastructural features to meronts. Synaptonemal complexes, suggesting that meiosis was involved during this sporogony, appeared in both nuclei of the sporont. The polar filament in the mature meiospore was anisofilar. Nine coiled turns of the polar filament were shown turning about the posterior portion of the spore. The polaroplast was composed of an anterior lamellate part and a posterior vesicular part. The anchoring disc was at the top of the straight portion of the polar filament. The meront of the lanceolate spore cannot be confirmed in this study. Two uninucleate lanceolate spores were eventually formed via nuclear dissociation. The polaroplast of the lanceolate spore was divided into an anterior multi-chambered part and a posterior of reticulate part. The polar filament was of the isofilar type, consisting of at least 5–6 coils. This parasite was extremely similar to the species Amblyospora trinus; in spore shape and development; but different in spore size and their hosts. Although it was similar to species of Amblyospora in host/parasite relations, we would rather assign this parasite to a newly established genus, i.e. Intrapredatorus and the name Intrapredatorus barri n.g., n.sp. was given for this microsporidium. The characteristics of the new genus was discussed in this article. However, its transmission routes remained uncertain thus far.