This study examines the diapause response of naturally occurring boll weevil populations in north Mississippi, using weevils collected throughout the growing season (shortly after oviposition in squares) and held under simulated natural environments of temperature and photoperiod. Results indicate that boll weevils enter diapause throughout the growing season, starting with the first generation in July. The percentage of weevils in diapause increases as the season progresses, achieving an average maximum rate of 98.7% late in the year. A greater proportion of males achieve diapause than females at any given time except late in the season, when convergence in the diapause response occurs between the sexes. Models are presented of percent diapause of males and females as a function of julian date of emer-gence, useful in predicting weevil diapause in Mississippi. More mechanistic models are presented of percent diapause as a function of daylength, which have potential application over a wide region of the cotton Belt.