A survey of water mite larvae parasitizing adult mosquitoes was conducted throughout Pennsylvania. In total, 929,873 individuals and 46 species of mosquitoes were collected from 6,499 sites, and mites were examined from a subset of the parasitized mosquitoes. From 282 of the sites, 1,836 mosquitoes were parasitized by 4,769 mites, with a mean intensity of 2.6, ranging from 1 to 31. Twenty-one species of mosquitoes representing Aedes, Anopheles, Coquillettidia, Culex, Ochlerotatus, Orthopodomyia, and Psorophora were parasitized by 1 Parathyas sp., 7 Arrenurus spp., and 7 Arrenurus morphotypes. All mite species are documented from Pennsylvania for the first time. The largest numbers of hosts were Coquillettidia perturbans and Ochlerotatus trivittatus, and of parasites were Arrenurus danbyensis and Parathyas barbigera. Aedes spp., Ochlerotatus spp., and Psorophora ferox were mostly parasitized by P. barbigera, Anopheles spp. and Cq. perturbans mostly by Arrenurus spp., and Culex spp. by both P. barbigera and Arrenurus spp. Thirty-three different associations were observed, and 17 of these are new records. Parasitism of individual mosquitoes by more than 1 mite species was rare. Most P. barbigera individuals were attached on the pre-abdominal region of their hosts, and, when not in this position, they were attached anterior on the thorax, and less commonly on the cervix or abdomen. Most A. danbyensis and Arrenurus delawarensis individuals were attached to the cervix of Cq. perturbans. Arrenurus danbyensis on Cq. perturbans had mean and maximum intensities of 2.8 and 31, and showed a clear trend in attachment site distribution, with sequential progression from head to abdomen.