Sixteen virus isolates with similar biological characteristics were obtained from salt-marsh mosquitoes collected in south Texas in 1974. When compared antigenically, these and 13 other isolates from mosquitoes collected between 1972 and 1979 in west Texas, New Mexico, Louisiana, Colorado and North Dakota were shown to be related but not identical. Three distinct serotypes were determined: Bahia Grande (prototype strain TB4-1054), Reed Ranch (TB4-222) and Muir Springs (76V-23524). When examined by electron microscopy, these three viruses were shown to be rhabdoviruses. Structural analysis of the prototype strain of Bahia Grande virus from Texas revealed five proteins. Comparative oligonucleotide fingerprint maps showed 51 to 86% sharing of the large oligonucleotides between Bahia Grande virus (strain TB4-1054) and 11 other antigenically related isolates but not with Muir Springs virus (strain 76V-23524), an antigenically distinct isolate from mosquitoes collected in Colorado. A serological survey for antibody to Bahia Grande virus showed that humans, cattle, sheep, reptiles and wild mammals from south Texas had neutralizing antibodies to this virus.