Measures of private speech and task performance were obtained for a sample of 46 5- and 6-year-olds engaged on a mechanical version of the Tower of London (ToL) task. Two different sets of four puzzles of increasing difficulty were attempted on two occasions. In line with Vygotskian predictions, there was a quadratic relation between private speech and task difficulty, but no evidence of a shift towards self-regulatory sub-types of private speech with increasing task difficulty. Levels of self-regulatory private speech were significantly related to concurrent, but not subsequent, task performance. We discuss the significance of these findings for the Vygotskian view that private speech has an adaptive function in the self-regulation of behaviour.