(Uploaded by Plazi for the Bat Literature Project) We performed the Þrst quantitative survey of ectoparasitic assemblages on three species of mormoopid bats living on the Caribbean island of Puerto Rico: Mormoops blainvillii Leach (n ϭ 40), Pteronotus quadridens Gundlach (n ϭ 40), and Pteronotus parnellii Gray (n ϭ 9). We examined bats for parasites primarily on 8 Ð10 May and 24 Ð27 July 2002 at Culebrones Cave, near Arecibo, Puerto Rico. Only 50% of M. blainvillii were infested with at least one parasite, compared with 85Ð100% for Pteronotus; infested individuals of both species of Pteronotus also harbored a greater number of parasites than did M. blainvillii. The assemblage on M. blainvillii was less diverse than in the other species and more dominated by a single group of parasites, the chiggers. Chirodiscid ear mites and spinturnicid wing mites were common on both species of Pteronotus but absent from M. blainvillii. All P. parnellii harbored streblid ßies, but none typically was found on P. quadridens or M. blainvillii. Adult female M. blainvillii and P. quadridens sheltered a greater number of ectoparasites than did males. All hostÐparasite combinations represented new associations for Puerto Rico.