Argyranthemum coronopifolium (Willd.) C. J. Humphries, a rare endemic of Ten–erife in the Canary Islands, has a disjunct distribution which coincides with the two largest Tertiary basalt massives, Teno and Anaga. No morphological differentiation was found between the Teno and Anaga populations. The disjunction is believed to be due to barriers caused by lava flows in the intermediate region of North Tenerife. A. coronopifolium is shown to be hybridizing with the widespread A. frutescens (L.) Schultz Bip. in both Anaga and Teno. The hybridization was analysed phenetically by Wells' distance method and principal coordinate analysis. Geographical barriers between the species are shown to be broken down by road side immigration of A. frutescens into the area of A. coronopifolium, and the possibility of swamping is discussed. The trend of increasing hybridization is connected to the development of tourism in the Canary Islands.