Three species of Mustelidae are found in New Zealand: ferrets (Mustela furo), stoats (Mustela erminea), and weasels (Mustela nivalis vulgaris). The introduction and spread of mustelids into a wide range of habitats different from those of their native lands has provided an opportunity to study the impacts that habitat differences might had on mustelid mandible morphology, especially stoats which are more widespread in New Zealand. Geometric morphometrics were used to make comparisons of the morphological variation of mandibles within and between the three New Zealand species. Each mandible had 24 landmarks. There was size sexual dimorphism within each species but no shape sexual dimorphism. However, there was between species allometry and mandible shape differences, which can be related to diet composition and the bite force required to kill prey. The second comparison examined the morphological plasticity of stoat mandibles collected across ten New Zealand habitats and one English location. There was no shape sexual dimorphism and the degree of size sexual dimorphism was different at each location. Male stoats had a high variation in mandible size likely from size plasticity in a response to differential prey availability during growth. Some locations had significantly different mandible shapes from others, these matched differences in biomechanical advantage and likely represents adaptation to the environment. Mandible shape of stoats was correlated with rainfall which has been correlated to mice density. My results also called the into question the correlation between mandible size and skull size, which now requires further study.