The objective of this study was to simultaneously evaluate the impact of leafy spurge (Euphorbia esula L.) on plant species richness within and among a wide variety of vegetation types typical of the region. The study was conducted in Theodore Roosevelt National Park in southwestern North Dakota where 11 plant associations were identified as being particularly susceptible to invasion by leafy spurge. Representative infested and non-infested stands of the 11 associations were sampled using the protocol described by the National Vegetation Classification System. To evaluate the effects of leafy spurge infestation across a variety of vegetation types, the 11 associations were grouped into 1 of 4 general physiographic/vegetative units (floodplain, grassland, woodland, and shrubland). Species richness per sampled infested stand was reduced by an average of 51% (P ≤ 0.05) in 7 of the 11 sampled associations. Total species richness in infested stands averaged 61% less than species richness within their non-infested counterparts for 10 of the 11 associations. Thirty species common to the majority of the associations were completely absent from infested stands and classified as sensitive, whereas 25 were minimally impacted and classified as persistent species. The overall effects of leafy spurge on species richness is complex and probably involves patterns of soil moisture, nutrient conditions, and disturbance that influence the abundance and distribution of all alien plants in Theodore Roosevelt National Park. However, because of the considerable redundancy in species distribution among a wide variety of plant associations and an aggressive control program, overall species richness does not appear to be threatened by infestations of leafy spurge at this time.