Infection levels of Polystoma marmorati (Polystomatidae: Monogenea) in the painted reed frog, Hyperolius marmoratus, and P. umthakathi in the kloof frog, Natalobatrachus bonebergi, were studied under natural conditions. The recorded prevalence for P. marmorati was generally high in adult frogs and low in tadpoles. In the case of P. umthakathi, the opposite was found with a low prevalence in adult frogs and a very high prevalence in tadpoles. Based on these findings, the hypothesis is presented that for P. marmorati the neotenic stage does not play a determining role and the population is maintained by bladder parasites. For P. umthakathi the neotenic stage appears to be more important than the bladder parasite and the population can probably survive without any significant contribution from bladder parasites.