A review of relationships between pesticidal chemicals and non-target animals living in the wild indicates that modern compounds, while generally beneficial, are having unexpected, unwanted adverse effects on some desirable species of fish and birds. This happens partly because pesticides may be carelessly used, and at times their acutely toxic effects are apparent. The majority of effects appear because some chemicals persist in the environment, become stored in animal tissues, pass through food chains, and affect behavior or survival of individuals and reproductive success of populations. Selective removal of predator species occurs and prey species increase in numbers. Some economic losses have resulted when commercially valuable fish or bird species have accumulated chemical residues to levels which made them unmarketable.