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Background: Temperature has a fundamental effect on organisms because it alters the speed of biochemical reactions and thus metabolism. This influence scales up to have ecosystem wide effects as the life history strategies of individual species differ in response to temperature. With the prospect of increasing global temperatures ecosystem functions could be interrupted. In order to predict the consequences of changing environmental conditions it is important to first establish how fitness related traits are affected by changing thermal conditions. Aims: The aim of this thesis was to develop a detailed understanding of the thermal niche of an invasive, tropical freshwater fish species. Methods: Using ecologically realistic conditions this thesis investigates the effect of environmental variation within and between generations on behavioural, growth, physiological, and reproductive characteristics of the Trinidadian guppy (Poecilia reticulata). Results: The results provide an insight into the fundamental thermal niche of a widely used model fish species as well as detailed measures of how thermal change alters phenotypic characteristics. Guppies are demonstrated to have a broad thermal tolerance and be phenotypically responsive to changing environmental conditions. The results also suggest that environmental characteristics of the guppy's habitat make an important contribution to the differences observed between populations of guppies in Trinidad. Conclusions: Water temperature in the guppy's natural environment varies widely over a daily cycle and I suggest that this is partly responsible for the guppy becoming phenotypically plastic and thermally tolerant. Furthermore, phenotypic flexibility is an important characteristic that will enable guppies to withstand some climate warming and continue to expand their invasive range poleward. Using experimental conditions which resemble those in the natural environment is important for developing accurate model parameters. These are necessary for predicting the ecosystem effects of environmental variation and for adaptive mitigation or pre-emptive management of range extensions by invasive species.