Information on an undescribed keroplatid fly (tribe Orfeliini) from Cameroon is presented. Its myrmecophagous larvae live in hollow stems of an understory myrmecophytic tree. Data on the biology of this fly and its interaction with one of the plant's strictly associated ant species Cataulacus mckeyi (Myrmicinae) showed that the number of keroplatid larvae per tree was not limited by occupation competition with the ant. Through predation on workers, keroplatid larvae may have a sizeable impact on colonies of C. mckeyi.