Within the “Wrocław Longitudinal Twin Study” in a part of the material the liking level of singletons (SIN) to identical twins (TMZ) and of TMZ to SIN was analysed cross-sectionally, in the period of 11th to 18th year of life. In each of the 76 classes of elementary and high schools in Wrocław (Poland), including the investigated in 1976-7 TMZ (64 pairs), were evaluated the relations between school-mates, using the Korczak's five-degree scale of liking. The declared by TMZ and SIN liking decreased with age between pupils of the same sex, and increased between pupils of opposite sex, however, in TMZ with some delay. The liking declared by TMZ of both sexes to SIN was on average lower than that declared by SIN to TMZ. The degree of liking declared by SIN to TMZ of the same sex was average higher than in relation of SIN to SIN particularly among girls. Thus, there occurred the so-called “prima donna effect”. The liking declared to pupils of opposite sex in relation of SIN to TMZ was on average lower than the analogous one in relation of SIN to SIN and in relation TMZ to SIN even worse. The higher contrast in treating pupils of the same and opposite sex in TMZ than in SIN indicates a lower social maturity in TMZ. A diverse influence of socio-economic conditions on the declared toward then liking by boys and girls was found. The low number of separated twins did not allow to make a generalisation on specificity of their position in class. The sending of TMZ to different classes appeared to be in Wrocław elementary schools sporadic (1 repeater), and in high schools observed in 38%, from that 28% of twins went to equivalent classes, usually in different school, and in 10% of pairs one of the twins repeated the class.