Designed to examine the nature of ideas that students hold about specific scientific concepts and to investigate modes of instruction that would effectively help them gain an accurate understanding of their world, this study focused on students' conceptions of ecological concepts and the influence of field instruction strategies on students' understanding and retention of these concepts. An experiential 7-day field program served as the learning strategy for three independent groups of secondary students. Students responded to a specially designed instrument, Student Ecology Assessment (SEA), prior to, during, and 4 weeks after the field program. Background data, instructional emphasis ratings, SEA concept subscores and total scores were entered into multiple regression analyses. All groups exhibited significant posttest gains (at the .001 level) and showed evidence of retention of the targeted concepts. Gains in scores in the major concept strands were positively related to the instructional emphasis given to those areas. The effectiveness of the field program was apparent in that specific concepts that were emphasized were learned and retained. The mastery approach of learning in a field setting proved to be successful. A 100-item bibiligraphy and 118 data tables are included. (ML)