To identify whether biology achievements change with the lime-of-day or not, we analyzed their scores, The classes were divided into morning/afternoon and boys/girls groups. Boys` achievements showed morning superiority, and the girls` showed afternoon superiority. In low- and average-achievement groups these patterns were similar to those of whole classes. These trends coincided with those of changes of verbal and arithmetic thinking abilities with the time-of-day. However, in high-achievement groups both boys` and girls` achievements showed morning superiority. These results suggest. that the chages of cognitive abilities with time-of-day should be carefully considered in time scheduling and in the designing and implementation of science instruction.