Many aspects in the life-history of aphids are critically dependent on the quality of their host plants and prevailing temperature. Therefore, the fitness of an aphid clone will depend on these parameters and will determine its ecological and ultimately its evolutionary success. Measuring and calculating the fitness of an organism in a natural environment is an important but also a difficult task, as many parameters that code for fitness need special assumptions, e.g. a uniform environment or stable age distribution. In this study, three aspects of environmental variability were considered: (a) the nutritional supply of the host plants (high- and low-quality plants), (b) the changes in host plant quality due to the endogenic life cycle of the host and (c) constant and variable temperature regimes. For each of three successive generations of Cinara pruinosa (Hartig) feeding on Picea abies (L.) Karsten, the change in fitness was determined by calculating the intrinsic rate of increase (rm) and expected total reproductive success (ETRS) when the aphids were reared under greenhouse (constant temperature) or field (variable temperature) conditions. Nutritional supply, plant life cycle and temperature affected the fitness of aphids, with fluctuating temperatures obscuring the effects. As a consequence, differences in fitness values among treatments were most pronounced under the constant temperature regime of a greenhouse and less marked in the field. If plant quality varies but not temperature, the contribution to clonal fitness of early generations is overestimated in comparison to later generations. The limitations and consequences for the interpretation of fit ness values of aphids are discussed.