HEEP COPYRIGHT: BIOL ABS. Many natural products show remarkable durability in natural ecosystems, and some of these substances are of practical concern. A variety of synthetic compounds (e.g. pesticides, polychlorinated biphenyls, synthetic polymers, surfactants, nitrogen compounds, industrial wastes) are also persistant in nature, and this resistence is important because of the large amounts of such chemicals now manufactured and their effects in a variety of environments. The degradation of synthetic chemicals results mainly from microbial activity, and attempts to minimize soil and water pollution would be facilitated by adequate information on why many pollutants are so refractory. Fifteen possible mechanisms of recalcitrance in natural systems are discussed. Biodegradability of a compound often can be increased by appropriate structural modifications. Manufacturing costs may be increased, but if society wishes to enjoy the benefits of synthetic chemicals and yet have an environment free of toxic or offensive materials, it must accept the additional cost.