While carrying out a research into schoolchildren’s ideas and knowledge about sex, the author obtained 1,506 inquiries from mixed classes of 13, 14 and 15-year-olds attending the last three forms of the Basic Nine-Year School. He sorted out the inquiries putting each of them into one of the ten groups adopted from a German work by Grimm and Rössler of 1950/56, with whose results he compared his own. 18 per cent of the total number of inquiries were left over and it was necessary to make a special group for them, for it was logically impossible to include them in any of the original ten groups. These inquiries were concerned mainly with moral questions occurring much more frequently among girls than among boys. Among the girls there appeared to be a stronger interest in their own development, in the problems of conception and child biology. The boys, especially in the last two forms of the Basic Nine—Year School, asked mostly about sexual intercourse. This is undoubtedly connected with the specific peculiarities of the time proportionality of biological and psychological changes of the two sexes in the period of adolescence, which make us feel the difficulty of collective instruction in this field.