Word association techniques were used to examine the growth of biological knowledge over a period of years, from fourth-grade to college students. Results were analyzed by classifying stimulus-response word pairs according to the nature of the relationship between the words in each pair. Three hypotheses were tested: (1) the proportion of enactive (verb) responses will be greatest among fourth-graders and will decrease with increasing knowledge of biology and with increasing maturity of subjects; (2) the pattern of word association responses in early adolescents (sixth-graders) will be different from that seen in younger and older groups, and will be consistent with concrete operational thought patterns as described by Piaget; and (3) as individuals learn more biology, the hierarchical organization of their knowledge will increase, as reflected by an increase of superordinate, subordinate, and definition responses. Results (based on response patterns) provide strong support for the Piagetian sensorimotor, concrete operational, and formal operational stages. They also reveal increasing hierarchical organization of knowledge with increased biology learning. A bibliography with 33 references concludes the report. (JN)