The purpose of this study was to analyze the errors made by high school students in practical biology and the extent to which boys and girls differ in terms of the frequency of errors committed. The subjects included 317 students from the twelfth grade sampled from five different schools in Nigeria. A criterion level of 0.05 was set to test the two null hypotheses: there is no significant difference in error types between schools and there is no significant difference in the frequency of error types committed by boys and girls in practical biology examinations. One-way analysis of variance and chi-square statistics were used to test these hypotheses. The results of the analysis showed that all the subjects committed certain degrees of errors which differ from school to school. These errors were found to be gender-related with girls committing less errors than boys. It is suggested that teachers should spend more time exposing students to possible errors during practical lessons in order to reduce the errors committed and thus improve the students' achievements. Contains 28 references. (Author/JRH)