Antifilarial activity of 5/6/7/8-mono- or disubstituted 1 H/1-phenyl-9H-pyrido[3,4-b]indoles (I) has been described. The 1,6- and 8-substituted 9H-pyrido[3,4-b]indoles (I) elicited interesting filaricidal activity against Litomosoides carinii and Acanthocheilonema viteae in rodent hosts.