Three viruses were isolated from ticks parasitizing nesting seabirds on Cape Sizun, Brittany, France, during the spring of 1979: Soldado like virus (Bunyaviridae, Nairovirus genus) from Ornithodoros (Alectorobius) maritimus (Argasidae), Zaliv Terpeniya (ZT) virus (Bunyaviridae, Uukuvirus genus) from Ixodes (Ceratixodes) uriae (Ixodidae) parasitizing chicks of the kittiwake (Rissa tridactyla), and a virus of the Sakhalin group from I. (C.) uriae. Certain virological and ultrastructural properties of ZT virus were studied and the antigenic relationships between th French isolate and other UUK serogroup viruses from I. (C.) uriae were analyzed in the complement fixation and immune diffusion tests. Cape Sizun is notable for the unusual presence of both argasied (Ornithodoros) and ixodid (Ixodes) ticks infesting seabirds in adjacent niches in a single locality, where they are infected by different viruses.