Naturally parasitized fish were collected and examined for glochidial infestations to evaluate host use by 2 mussel species, the yellow lampmussel (Lampsilis cariosa) and tidewater mucket (Leptodea ochracea), that are listed as threatened in Maine. Fish were captured at 13 sites in 3 river watersheds throughout the range of L. cariosa and L. ochracea in Maine to determine if host fish identified in the laboratory are used in nature and to assess additional species as potential hosts. A species-specific molecular identification key using restriction fragment length polymorphism patterns of the mitochondrial NADH dehydrogenase subunit 1 gene was used to identify 687 glochidia from 230 fish. White perch (Morone americana) and yellow perch (Perca flavescens) were previously identified as host fish for L. cariosa in laboratory trials and were confirmed as host fish in wild populations. Five additional species were identified as potential hosts for L. cariosa. Three of these potential hosts are native (banded killifish [Fundulus diaphanus], chain pickerel [Esox niger], and white sucker [Catostomus commersoni]), and 2 are introduced (smallmouth bass [Micropterus dolomieu] and largemouth bass [Micropterus salmoides]). For L. ochracea, the host status of white perch was confirmed in wild populations, and 1 additional species, the banded killifish, was identified as a potential host. White perch was the most frequently and heavily parasitized host; several fish in multiple sites were heavily infested with L. cariosa or L. ochracea glochidia. In contrast, only 1 individual of each of the other fish species, including yellow perch, was infested with L. cariosa or L. ochracea glochidia. We present data collected incidentally for confirmed or potential host fish for other mussel species in the study area. These species include Elliptio complanata, Lampsilis radiata, Anodonta implicata, and Alasmidonta undulata.