The author ascertained various personality signs and traits by means of questionnaires filled in by class teachers about their pupils. Each age group included 95 boys and 95 girls. The author tested reliability in each age group by comparing odd and even results by means of X2 values. Agreement was statistically significant everywhere on the level of 5 per cent. Through comparing results and also by means of X2 values the author was trying to find out statistically significant differences between boys and girls in each age group. Boys proved to be livelier, more restless and had more conduct offences than girls. Girls, on the other hand, proved to have a more sensitive approach towards people, to be more active and painstaking during classroom instruction and had a better attitude towards mental work (learning). Another thing that appeared was that children’s mothers visited the school more frequently than fathers and also took better care of their children’s appearance than fathers did. Development curves of the children’s personality traits have two peaks of maximal occurrence of a sign one of them at junior school age, the other at middle school age. Teachers do not have uniform criteria for the appraisal of pupils in different age groups. Teachers appraised pupils from 10 to 12 years old as being furthest below average from the point of view of knowledge and intellectual level. In this age group they overrated girls, whom they appraised more often as being above* average, and underrated boys, whom they labelled more frequently as below average. This work may be regarded as a contribution to the theory of knowledge in the field of development psychology, where age distinctions between boys and girls have not everywhere been taken account of so far and where the course of development of some personality signs is disregarded altogether.