The extent and direction of curriculum bias in standardized reading achievement tests are examined. Bias is estimated by comparing the relative overlap in the contents of five separate reading achievement tests with the content of seven commercial reading series at first and second grade levels. Overlap between each achievement test and each reading series is reported in terms of achievement test grade equivalent scores that would be expected given mastery of the words which appear both as content in a reading series and as achievement test items. Results indicate clear discrepancies between the grade equivalent scores obtained both between tests for a single curriculum and on a single test for different reading curricula. The implications of the apparent curriculum bias of achievement tests are discussed as they relate to teacher, child, and curriculum evaluation, to reading replacement, and to applied educational research.