Introduction
Tilapia zillii has a wide distribution in lakes and rivers of Africa (BOULENGER, 1951) . In certain areas in Africa, especially in Congo, Tilapia zillii is the most abundant species, being kept also in ponds (HUET, 1957) . In Israel, Tilapia zillii is one of four species of Tilapia found in all streams and in Lake Tiberias . Tilapia zillii in Lake Tiberias is different in appearance from the same species in Dor, Fish Culture Research Station. The source of this group of fish is the Diphli river.
The aim of this work is to define the differences between the morphological characters of the two groups, and to find, by crossing the above types, whether those differences are phenotypic or genotypic.
Methods
During February and March 1965 fish caught with ring nets in Lake Tiberias were examined . Most of the fish were measured in Tiberias, and live ones were transferred to ponds in the Fish Culture Research Station, Dor, and put into holding ponds . At the same time Tilapia zillii from the commercial ponds in Dor were examined .
Differences in source materials
The fish from Lake Tiberias are different in shape and colour from those of Dor. The differences between the two groups are shown in Table I .