Twenty-four children aged five and twenty-four children aged six were interviewed individually three times during a calendar year. It was found that not only did the children's language develop over the period, as judged syntactically and lexically, but they also showed an increasingly fluent control over their own style. All the children exhibited, however, certain common factors in their linguistic performance which correlated with factors other than their grammatical competence, namely, the presence of certain features in the situation of the interviews. These non-linguistic features were isolated as: the task set for the children; the topic they were asked to discuss; and the conceptualization by the child of the role of the listener. Although largely ignored by research on child language, situational factors may be very important for assessment, where not only linguistic performance but linguistic competence is concerned. They are important for development theories because where a child uses his best language is a clue as to where language is acquired. Finally, situational effects are important for the design of educational programs because they suggest how we can facilitate the child's talking and his talking in his most advanced language. (Author/AM)