To study the behavioral style or temperament of 45 boys, aged 47 to 88 months, with full-mutation fragile X syndrome (FXS), 102 parent ratings on the Behavioral Style Questionnaire (McDevitt and Carey 1978) were recorded. These ratings were analysed with a variety of statistical techniques. Considerable variability was evident in temperament profiles; consequently, a characteristic profile was not identified for FXS. Boys with FXS differed significantly from the reference sample on five of nine temperament dimensions. They were more active and less intense, approachable, adaptable, and persistent. No significant differences were found in distractibility, rhythmicity, mood, or sensory threshold. Only 16 of the 45 boys in the sample could be classified as easy, difficult, or slow to warm up. There was no link between severity of developmental disability and temperament ratings. This supports the theory that intelligence and temperament are separate constructs. Scores on temperament dimensions were stable over time. Our results suggest that many of the behaviors observed in boys with FXS may be related to temperament. Consequently, parent counseling and environmental modifications should be considered as first line treatment. The question of whether the behavior problems observed in boys with FXS are innate or whether they result from poorness of fit between child and environment is an important issue that needs further study.