The use of epidemiological studies of twins as a means of determining the existence of genetic factors in various conditions is well known. To be of value, the zygosity of the twin pairs must be identified with as great a degree of confidence as possible. A method of determining twin zygosity, using concordance in general likeness, hair colour, hair type, eye colour, ear form, tongue roll and phenylthiocarbamide (PTC) taste sensitivity, was developed for use in a survey of 244 pairs of like-sexed twins aged 5 to 15 years. The results obtained and the problems raised when applying this method are discussed. The method was simple to apply to a large group of twin pairs, and monozygotic pairs were identified with a degree of confidence greater than 95%.