Clear-cut variation in the length of genital filaments amongst Bolivian specimens of Lutzomyia runoides (Fairchild & Hertìg, 1953), of the species group aragaoi, allowed us to recognise two different, non-overlapping populations living in sympatry. The additional observation of their mutually exclusive colonisation of isolated trees strongly suggested the existence of cryptic speciation. Multivariate, size-in and sizefree analyses bearing on characters other than genital filaments could separate the two putative cryptic species of L. runoides, and showed that they were different from L. inflata, previously regarded as conspecific.