Background/Question/Methods Parasites are often spatially aggregated within host tissues and organs, but the mechanisms responsible for this pattern and how it arises are often unknown. Such asymmetry in infections could result from variation in the quality of habitats within the host, parasite aggregation or competition, or as a result of the coevolutionary history of host and parasite. In the current study, we sought to investigate whether the occurrence of Echinostoma metacercariae exhibit biased distributions between the left and right kidneys of larval amphibians and to explore potential mechanisms underlying such patterns. We necropsied recently metamorphosed amphibians from 23 National Wildlife Refuges to quantify the distribution of helminth parasites with a focus on echinostome infections in the kidneys. We examined 265 individual amphibians representing 10 species that exhibited infection intensities of >10 Echinostoma metacercariae. Results/Conclusions Using hierarchical stochastic models fitted by maximum likelihood, we found that Echinostoma was usually more abundant in the right than left kidney for all species across all sites (proportion in right 0.58 ± 0.011 standard error). However, the degree of asymmetry varied among individual hosts, ranging from 0.41-0.64 of parasites in the right kidney. Among the three amphibian families observed (Bufonidae, Hylidae, and Ranidae), the greatest degree of right side bias occurred in Hylidae (64% ± 7.4 standard error). The degree of asymmetry also varied among host species. Within the two most abundant anuran species examined, metacercariae in Pseudacris regilla (Hylidae) showed a high degree of right-side bias (61% ± 1.7 standard error, N=140), whereas parasites in Lithobates (=Rana) catesbeiana exhibited no bias (50% ± 1.7 standard error, N=42). The proportional difference between right and left kidney length was low (3.5% ± 0.2 standard error N=187), which suggests differences in kidney length are unlikely to account for observed patterns. These results clearly show overdispersion and suggest a combination of processes including host morphology and physiology and parasite behavior. Our study identifies potential ecological mechanisms for biased parasite distributions within hosts that can be examined further in other natural systems.