Protopolystoma xenopodis and Protopolystoma orientalis are polystomatid monogeneans respectively specific to the parapatric anurans Xenopus laevis and Xenopus muelleri. Parasite larval stages may invade the kidneys of foreign Xenopus spp. but die before migration to the definitive urinary bladder site. Laboratory experiments to assess the effect of a primary incompatible kidney infection on a secondary compatible infection found: (1) a small, significant decrease in the survivorship of P. xenopodis kidney stages (23-37 days p.i. at 25°C) in X. laevis laevis previously challenged with P. orientalis; (2) a significant effect of prior P. orientalis challenge on P. xenopodis development and establishment in the urinary bladder of X. laevis 100 days p.i. (at 21°C); (3) no effect of prior P. xenopodis challenge on adult P. orientalis establishment in X. muelleri (at 21°C), but a significant negative influence on reproductive output (days 0-50 post-patency). Partial cross-resistance to heterospecifics may therefore be induced by Protopolystoma spp. infections in the kidneys of an incompatible host, demonstrating that at least some elements of the host response are non-species specific. The effects observed were weak compared to the strong host resistance known to be generated by an established compatible primary infection with respect to conspecifics. This difference suggests that strong acquired resistance to Protopolystoma species is species-specific and/or induced only by older stages surviving in compatible hosts.