On islands across the Pacific the invasion of the gecko Hemidactylusfrenatus has caused a decline in the abundance of a resident gecko, Lepidodactyluslugubris. In a previous study we demonstrated that the prevalence of the cestode Cylindrotaenia sp. is higher in the resident gecko on islands where it is sympatric with the invader than on islands where it occurs alone. In the present study we experimentally test whether the presence of the invading gecko causes an increase in parasites, particularly Cylindrotaenia sp., in the resident. In addition, we test whether the effect of the invader on parasite prevalence in the resident is mediated through an increase in corticosterone in the resident. Corticosterone is the primary glucocorticoid, or “stress” hormone in lizards, and chronic elevation in corticosterone may suppress some types of immune responses. After experimental manipulations of interspecific interactions (single vs. mixed species treatments) and intraspecific densities (high vs. low), we detected no difference in parasite prevalence or circulating corticosterone among the experimental treatments in either L. lugubris or H. frenatus. Circulating levels of corticosterone were higher in geckos␣sampled at night than geckos sampled during the day, indicating a circadian cycle in corticosterone levels in these nocturnal animals. Circulating levels of corticosterone were higher in experimental geckos than in geckos that had not been used in the experiment, and, in some groups, higher in geckos infected with cestodes than in uninfected geckos. Circulating levels of corticosterone did not differ between non-experimental H. frenatus and L. lugubris, but when geckos used in the experiment were compared, circulating levels of corticosterone were significantly higher in H.␣frenatus than in L. lugubris.