Studies were made on the relationship between Trichuris suis of the pig and T trichiura of man. Comparative morphological and developmental studies revealed differences in sizes of the eggs and infective first stage larvae, in the rate of development to the infective larval stage and in development within young pigs following experimental infection. Successful cross infections of man with T suis ova and of pigs with T trichiura ova were achieved for the first time. T suis eggs recovered from one human experimental infection were used to produce a patent infection in a pig. It is argued that while the successful cross infections suggest a close evolutionary link, the differences in morphology, exogenous and endogenous development provide sufficient criteria for the two parasites to be maintained as separate species.