The distribution patterns of three subspecies of butterflyweed (Asclepias tuberosa) are indicated, and a canonical correlation has been used to relate various morphological characteristics within them to selected environmental variables in the state of Oklahoma, U.S.A. Introduction and methods The spatial distribution of morphological variability in butterflyweed (Asclepias tuberosa L. subsp. interior Woodson) in Oklahoma may be related to environmental parameters. Asclepias tuberosa was studied by Woodson (1947), who described three subspecies distinguished on the basis of leaf characteristics. The most widely distributed of these three subspecies, A. t. interior, is found in an area extending from upstate New York, through the midwest and on into the southwestern U.S.A. The other two, A. t. tuberosa and A. t. rolfsii, are distributed in smaller areas in the southeast and southern portions of the country respectively. In a later paper, Woodson (1964) observed that the flower colour of the subspecies A. t. interior progressed quite systematically from a dark orange in the eastern portions of Oklahoma to a light yellow in the west. He suggested this might be related to the distribution of rainfall, but this was not tested. The distribution of these three subspecies in the U.S.A. and the colour of samples collected by Woodson (1964) are shown in Fig. 1. The shift in flower colour appears to correspond to a major ecotone and a noticeable change in environmental conditions in central Oklahoma. To the west, there is tall-grass prairie of the western grassland biome, populated by dominant species such as Andropogon gerardi (Big blue stem) and Bouteloua curtipendula (Side-oats gramma). To the east, a forested region is dominated by members of the Quercus-Carya association (Oak-hickory) of the eastern forest biome. Two crucial environmental factors, precipitation and soil type also vary across the state. Precipitation declines systematically from east to west, from an annual average of almost 1250 mm near the border with Arkansas to about 500 mm on the western border with Texas. With the exception of one area in the northeast part of the state, soils are essentially acidic to the east and basic to the west (Fig. 2). One of the objectives of this study is to establish an association between the flower colour of butterflyweed and precipitation. However, because of the evidence of colour variation and the fact that butterflyweed ranges between quite different biomes and environmental conditions, it was decided to attempt to relate a number of rnorphological characteristics to a broader set of environmental variables. These variables are listed in Tables 1 and 2. The morphological characteristics include plant length, internodal distance, leaf, bract, pedicel, sepal, petal, hood, stem hair length, petiole hair length and flower colour. The environmental variables consist of climatic, soil and slope indices. Climatic parameters include temperature, precipitation, humidity and drought frequency. With the exception of average annual maximum temperature, mean annual high temperature frequency, mean annual freezing temperature frequency and average proportion of months with severe drought, the majority of these are indicators of normal daily or seasonal conditions. Soil (SOL.) is represented as