A cytological analysis of the thelytokous speciesPsychoda parthenogeneticahas been made. A 3n=15 complement and a type of apomictic thelytoky have been found. Apart from the triploidy, the complement ofP. parthenogeneticais very similar to the one ofP. alternataandP. cinerea.The phylogenetic interrelationships with the related bisexual species P. severini and the origins both of the thelytoky and triploidy have been discussed.