The relationships between mean intensity and prevalence of infestation of Pseudione humboldtensis and the size and sex of the hosts, the squat lobsters Cervimunidia johni and Pleuroncodes monodon, from three fishing grounds in northern Chile are described. The prevalences were 13.6% and 6.8% for C. johni in Coquimbo and Huasco, respectively, while in P. monodon they were 18.2% and 0.6% in Coquimbo and Caldera, respectively. Between fishing grounds, no significant differences were found in the prevalence of infestation between male and female C. johni, but in Coquimbo, the prevalence of infestation in P. monodon was significantly higher in females. The sizes of infested squat lobsters were significantly lower than those of uninfested hosts. Males and females parasitized by P. monodon showed significantly lower body weights than uninfested hosts. Parasitized males of C. johni showed lower body weight than uninfested males. Only 0.6% of ovigerous females of C. johni and none of P. monodon were infested. The near absence of infested ovigerous females suggests castration process in females. A nonlinear relationship was detected between host size and parasite size for both females and males. Multiple infestations of P. humboldtensis were found in both host species, but they were more common in P. monodon. Given the high prevalence of multiple infestations (50%) and the significantly lower body weights of infested P. monodon, we suggest that this host is more vulnerable to P. humboldtensis than is C. johni.