This study validated the four mathematics tests of the Spanish version of the Woodcock-Johnson III (WJ-III) Achievement (ACH) battery for use in the first six grades of school in Spain. Developmental effects and gender differences were also examined. Participants were a normal population sample of 424 (216 boys) children aged 6 to 13 years. Results showed that the tests have good test-retest and internal reliability and good construct and criterion-related validity. Significant main effects of schooling were obtained with scores increasing across the six school grades, but scores between fourth and fifth graders did not differ significantly. Overall, boys scored higher than girls on all tests but the effect sizes of these gender differences were small (d less than or equal to 0.12). (Contains 4 tables and 1 figure.)