The occurrence and frequency of sympatric speciation in natural systems continue to be hotly debated issues in evolutionary biology. This might reflect the timescale over which evolution occurs resulting in there being few compelling observations of the phenomenon (lake fishes, phytophagous insects and Island trees). Despite predictions, few examples of sympatric speciation have been recorded in animal parasites, at least widely accepted as such. Here we show that, in New Caledonia, the monophasic (exploiting one individual host per generation) cattle tick Rhipicephalus microplus has evolved in contact with two sympatric host species into two differentiated genetic pools: on the cattle, its original host and on rusa deer, a new host for this tick. This sympatric isolation has occurred over a relative short period of time (i.e. less than 244 tick generations) as a consequence of differential selection pressure imposed by hosts. It is most likely that this phenomenon has occurred in many other places across the globe where this tick has come in contact with different host species in sympatry with cattle.