Resumo. Abstract: Aim Oomycetes are one of the most widespread and destructive parasitic groups in the world. The first infection of oomycetes on copepod Parabroteas sarsi Daday 1901 was recently recorded in America. Oomycetes infection on this species makes their eggs unviable and could differentially affect the survival of females. The aim of this study was to track the incidence of oomycetes in two populations of P. sarsi. Methods Two Patagonian ponds were monitored during spring-summer of 2014-2018, measuring environmental variables and the prevalence of oomycete infections on P. sarsi populations. Taxonomic determination of the oomycetes was performed in laboratory. Sex ratios also were recorded in each survey. Results The presence of Aphanomyces ovidestruens Gicklhorn 1923 and Pythium flevoense Van der Plaats-Niterink 1972 was detected in females of P. sarsi in both ponds. We observed a decrease in the parasite prevalence over time, from ~25% (assessed on the total number of adults) to almost disappear completely. Besides, a lower proportion of females than males were observed in samples with high prevalence of parasitism. No temperature differences were found during the sampling years and the monthly temperatures between the maximum and minimum prevalences were similar. Conclusions Our study presents a new record of oomycetes parasitizing copepods in America. Temperature can play a key role in the dynamics of parasite species, but in our study this variable not affected the incidence of the infection. However, other variables could be affect the pattern of the infection such as the presence of predators of oomycetes spores (cladocerans). The lower females proportion when the infection was higher, could support the idea of differential sex/mortality induced by the infection. Recording and identification of parasites is a critical starting point for wildlife management, which will provide an early warning to prevent the spread of harmful parasites such as oomycetes.