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Florida is home to the largest number of invasive reptile species, including the abundant Anolis sagrei, of any U.S. state. This lizard species has largely displaced the native Anolis carolinensis because of its superior competitive ability and tolerance of urbanized habitats. One facet of the invasion of A. sagrei that has not received attention in the literature is its interactions with parasites, including the malaria protozoan Plasmodium. We studied the relative prevalence of Plasmodium floridense in two Anolis species at three sympatric sites in central Florida to determine the factors that affect the parasite's prevalence, incidence, and effects on blood cell counts. After catching lizards and making blood smears, we examined slides for the presence of Plasmodium and counted relative erythrocyte/ leukocyte ratios. Of the 101 lizards sampled, 31% of A. carolinensis were positive for P. floridense, as opposed to only 11% for A. sagrei. Date of capture was also an important factor, with June having the highest prevalence and March the lowest. Erythrocyte to leukocyte ratios differed with infection, averaging 3.15 ± 1.28 SD in infected lizards versus 4.28 ± 1.03 in uninfected lizards. Reduced infection by blood parasites may have been one of the many factors that allowed A. sagrei to successfully invade Florida and to become the superior competitor throughout its range. Additional studies of the effects of Plasmodium infection on competitive ability and fitness are needed to determine whether infection is a major contributor to invasion success. [ABSTRACT FROM AUTHOR]