Embryonated chicken eggs were used as a model for assessing the teratogenic potential of several Palyam serogroup orbiviruses. Infection of 4-day-old embryonated chicken eggs via the yolk sac with eight of the viruses resulted in deaths or congenital deformities which included retarded development, arthrogryposis and reduced feathering. Statistical analysis showed that the viruses could be divided into three groups: those that caused death (Gweru virus isolates 866/77 and 1726/76; and Apies River virus), those that caused deaths only when large amounts of virus were inoculated (Gweru isolate AR11869 and Marondera virus) and those that caused death and deformities (Abadina, Kasba, Nyabira, Petevo and Vellore viruses). Differences in pathogenic potential were noted between isolates identified as the same serotype by serological tests.