The present study aimod at investigating development of friendship relations in adolescent twins. i) A qtiestionnaire, constructed to examine tho differences between friendship relations of twins and non-twins, was administeted to 160 identical twin and 420 non-twin adolesccnts (7 th-to 12 th graders). ii) 14 female twin and 8 non-twin subjects were required to rate personality charactoristics of their best friends, the 2nd best friends, ordinary friends, the other member of the twin pair and themselves, using SD adjectives. iii) 10 female twins and 5 non-twins were interviewed. They were encouraged to talk freely about their r.elations to friends and their Leelings about them. The results were as folloltrs : i ) Friendship relations of adolescent twins were not so intimate or important as in non-twins. This tendency was clearer ainong fcmales than males. Especially, in senior high school students, for whom intimate friendship plays an imp.ortant role in their psydhic life, there was marked retardation of developmont of twins friendship. ii) In twins, not orily was development of friendship with the bcst friend rotarded, but relations with the second best friend or to ordinary friends were more severely underdeveloped. iii) Thore were Lound two diLLercnt typos among twins whose friendship was undordevoloped. In one of them the role of friend was not clear and not differentiated from that of sibling. In this type the partner (i,e, other twin) could be a friend as well as a sister. In the other type interpersonal relations in general were cool and superficial.