Seed surface area is the basic ground for lint production in cotton and, hence, a starting point for genetic improvement in lint yield. Experiments on upland cottons were carried out under 2 temperature regimes (April sown, high temperature regime; June sown, moderate regime) for 2 years in the field to study their modifying effect on phenotypic expression and combining ability of lint weight per seed (LW/S), lint weight per unit seed surface area (LW/SA), lint weight per unit fibre length (LW/FL), lint frequency (LF), number of spinnable fibres per seed (F/S), and number of spinnable fibres per unit seed surface area (F/SA). Variation among cultivars for relative decrease in the basic lint traits under high temperature regime indicated their differences in heat sensitivity. Genetic variation for all basic lint traits was low over environments but high within environments. Temperature regime was a stronger source of variation in basic lint traits than year. Temperature regimes modified phenotype, ranking among parents, and combining ability of basic lint traits. Relative contribution of specific combining ability to total variation decreased under high temperature regime for all the basic lint traits, except LW/FL, with a corresponding increase in general combining ability due to either female or male parents. Relative contribution of general combining ability due to female parents for LW/S and LW/SA, and that due to male parents for LW/SA, LF, F/S, and F/SA, increased substantially under high temperature regime. High temperature regime was favourable for the expression of additive genetic variability. From the breeding point of view, F/S and F/SA were more useful traits.