The isolation barriers restricting gene flow between populations or species are of crucial interest for understanding how biological species arise and how they are maintained. Few studies have examined the entire range of possible isolation barriers from geographic isolation to next generation hybrid viability. Here we present a detailed analysis of isolation barriers between two flowering plant species of the genus Petunia (Solanaceae). Petunia integrifolia and P. axillaris feature divergent pollination syndromes but can produce fertile hybrids when crossed in the laboratory. Both Petunia species are primarily isolated in space but appear not to hybridize in sympatry. Our experiments demonstrate that pollinator isolation is very high but not strong enough to explain the absence of hybrids in nature. However, pollinator isolation in conjunction with male gametic isolation (i.e. pollen-pistil interaction) can explain the lack of natural hybridization, while postzygotic isolation barriers are low or non-existent. Our study supports the notion that reproductive isolation in flowering plants is mainly caused by pre- rather than postzygotic isolation mechanisms.