Forty-seven specimens of Neotropical cormorants, Phalacrocorax brasilianus, from Lago Guaíba, Guaíba, State of Rio Grande do Sul, Brazil (30° 00' S, 51°15' W), were examined for helminth parasites between 1999 and 2003. Twenty species of helminth parasites were found: ten digeneans: Austrodiplostomum mordax, Austrodiplostomum compactum, Clinostomum sp., Drepanocephalus olivaceus, Drepanocephalus spathans, Hysteromorpha triloba, Ignavia olivacei, Paryphostomum segregatum, Prosthogonimus ovatus, and Ribeiroia ondatrae; one cestode: Paradilepis caballeroi; eight nematodes: Contracaecum rudolphii, Eucoleus contortus, Eustrongylides sp., Ornithocapillaria appendiculata, Syngamus sp., Syncuaria squamata, Tetrameres (Gynaecophila) sp., and one undetermined capillariid (genus and species); and one acanthocephalan: Andracantha tandemtesticulata. The length and weight of male and female birds, as well as their sexual maturity (juvenile or adult), did not show significant difference regarding the helminth fauna; the standard length did not influence the helminth parasite indices. The prevalence of I. olivacei was higher in larger birds while the intensity of infection by this digenean species was higher in females. The abundance of P. caballeroi was higher in male birds. A. mordax and H. triloba showed higher prevalence and abundance in juvenile hosts, while O. appendiculata was more abundant in juveniles. The remaining species did not have their parasite indices influenced by the host parameters studied. The present work records the richest helminth fauna for any bird of the genus Phalacrocorax and is the first study to evaluate the influence of length, weight, sex, and maturity state on parasitism.