A total of 21 Colocasiomyia species, including 17 undescribed species, are reported from Sabah (Mt. Kinabalu and neighboring areas), Malaysia, based on samples collected from inflorescences of 14 or 15 Araceae species. This number of species is the largest as a local fauna of this genus in the world. The high species diversity is attributed to 12 undescribed species belonging to the Colocasiomyia baechlii species group. A particular breeding habit of Colocasiomyia is sharing of the same inflorescence by a pair of species, with partial niche separation between them: one species uses exclusively the pistillate (lower female-flower) section of the spadix for oviposition and larval development, whereas the other mostly uses the staminate (upper male-flower) section. However, the baechlii group species show quite different patterns of host plant use: many (up to eight) species cohabit on the same inflorescence. It is unlikely that they separate their breeding niches micro-allopatrically within an inflorescence. Instead, species composition and their proportions of individual numbers vary among different localities, seasons and host plants, with partial overlap among them. Such partial separations in local distribution, phenology and host selection would in combination contribute to their coexistence and promote the species diversity of this group.