Simple SummaryOvine gastrointestinal nematodes (GIN) have flexible life history strategies, meaning they can alter the relative energy invested into each life trait in order to optimize their fitness under different environmental contexts. Some of the most prevalent ovine GIN species, such asHaemonchus contortus, have pervasive global distributions, and it is not known whether distinct isolates respond to the same environmental challenge with uniform alterations to their life history strategies or if different strategies affect different fitness outcomes. To investigate this further, this study compared the life history traits (i.e., establishment, fertility, egg-larvae development) and experimental fitness of threeH. contortusisolates, following challenge in the parasitic (resistant vs. susceptible sheep) and free-living (summer, spring and winter climates) phases. The findings showH. contortusexhibit isolate-specific life history strategies to effectively maintain their fitness in spite of the environmental challenges. Partial exploration of the isolates’ transcriptomes further supports disparate expression profiles between them. These results bring new insights into the mechanisms by which GIN sustain their fitness across fluctuating environments, the results of which carry important implications for the sustainability of control interventions and in the potential comparability of experimental research.AbstractGastrointestinal nematodes (GIN) use flexible life history strategies to maintain their fitness under environmental challenges. Costs incurred by a challenge to one life trait can be recouped by increasing the expression of subsequent life traits throughout their life cycle. Anticipating how parasites respond to the challenge of control interventions is critical for the long-term sustainability of the practice and to further ensure that the parasites withstand favourable adaptive responses. There is currently limited information on whether distinct populations of a GIN species respond to the same environmental challenge in a consistent manner, with similar alterations to their life history strategies or comparable fitness outcomes. This study compared the life history traits and experimental fitness of three distinctHaemonchus contortusisolates exposed to environmental challenges at both the parasitic (i.e., passage through resistant or susceptible sheep) and free-living (i.e., exposure to diverse climatic conditions) life stages. The key findings show thatH. contortusmaintain their fitness under challenge with isolate-specific alterations to their life history strategies. Further, partial exploration of theH. contortusisolates transcriptomes using cDNA-AFLP methods confirmed disparate expression profiles between them. These results bring fresh insights into our understanding of the non-genetic adaptive processes of GIN that may hinder the efficacy of parasite control strategies.