Culex pipiens (Linneaus, 1758) are important arboviral disease vectors consisting of two morphologically indistinguishable forms, pipiens (L.) and molestus (Forskål, 1775), which differ in behaviours such as host preference and expression of autogeny. Hybrid populations are hypothesised to possess intermediate behaviours that are key drivers of disease transmission. In this thesis, the influence of hybridisation and larval habitat on the expression of phenotypic traits by Cx. pipiens were assessed through a combination of field collections and laboratory experiments with colonised mosquitoes. Extraction of trace DNA from individual pupal exuviae was developed to permit accurate biotype identification of live colony and field Cx. pipiens, achieving 100% PCR amplification success within 12-hours of eclosion, facilitating creation of single-biotype colonies for use in subsequent experiments. Single-pair and grouped mating experiments revealed that mating success, larval development time and larval survival varied between homologous and reciprocal crosses of colony Cx. pipiens forms, which could influence hybridisation capability under natural conditions. Phenotypic plasticity of Cx. pipiens was also demonstrated from six field populations and under varying larval nutritional availability. Characterisation using CQ11 identified that field-derived aboveground populations are dominated by the pipiens form (98.8%) whilst occurrence of molestus (0.4%) and hybrids were low (0.8%). Full microsatellite analysis revealed differentiation between UK mosquitoes and autogenous molestus samples from Sweden, whilst no discernible grouping according to CQ11 could be determined within UK samples. Overall, whilst colony and field Cx. pipiens demonstrated physiological traits capable of facilitating virus transmission, such as a lack of autogeny and production of viable hybrid progeny, barriers to hybridisation were also identified. Further, low levels of natural hybrid populations suggest biotypes are largely isolated under current conditions. Finally, microsatellite analysis of field and colony individuals questions the reliability of single molecular markers for accurate biotype delineation, demonstrating these are not indicative of phenotype.