Summary1. Adaptation and loss of genetic capacity differ chiefly in that adaptation is goal‐ directed whereas loss of genetic capacity is not. Given sufficient information about an individual organism and its environment, adaptations are recognizable without reference to historical events extending beyond a single generation. This is not true of loss of genetic capacity, which requires a preliminary judgement that genetic information now absent was present in ancestral organisms. Together, adaptation and loss of genetic capacity are the major contributors to overall reproductive fitness. Accidental selection is genetically associated with adaptation, but is not goal‐directed.2. Adaptations arevariant or invariant; invariant adaptations comprising biochemical unity, and variant adaptations contributing to biochemical diversity. Variant adaptations may be either exploitive or epigenetic. Exploitive adaptations are a measure of thegenetic capacity for phenotypic response to an altered environment, which the individual may not in fact encounter. Epigenetic adaptations are more rigidly programmed and are responsive to altered environments only insofar as these are a constant feature of the life cycle.3. Selected observations in the biochemistry of helminth parasites are examined with respect to their interpretation in terms of adaptation, loss of genetic capacity and accidental selection. Secure judgements concerning adaptation are often possible at the most general level, i.e. when the physicochemical properties of the environment, such as temperature or oxygen supply, are clearly defined. I t is more difficult to make judgements concerning the specific mechanisms used in achieving these goals. Conclusions concerning loss of genetic capacity require knowledge of the specific function through‐out the life cycle. In many cases loss of genetic capacity is only apparent, as the function appears in another part of the life cycle. Such apparent losses are in reality epigenetic adaptations. These concepts are helpful in interpreting past work and in devising new experiments.4. Development in helminth parasites includes a pronounced capacity for the orderly release of information to be used in the next stage. As each stage may require a radically different environment, programming for it may lead to phenomena which are superficially puzzling, such as the existence of aerobic electron transport systems in a stage whose energy metabolism is fermentative. The concept of epigenetic adaptation is especially useful for interpreting such observations.5. Although possible adaptations are most readily apparent in biochemically complex mechanisms, these mechanisms are an expression of the orderly effects of many different primary gene products which have not been much studied. There are indications that organisms possessing relatively complex life cycles may provide opportunities for relating primary gene products, such as isozymes, to their physiological func