Gynodioecy is 8 breeding system consisting of male, steriles (MS, females) and hermaphrodites (H). There is however within such systems a third, often neglected, class of partially male sterile plants (PMS), i.e. plants with an intermediate sex expression, In natural populations of Plantago coronopus partially male sterile individuals were us common as male steriles (frequencies in the range 1·7–36·8%). These PMS plants constitute a continuous series, linking complete male sterility and hermaphroditism. We discovered that temperature influenced sex expression and the degree of male sterility of PMS individuals, but not of MS and H individuals. Thus, at higher temperatures PMS individuals became more male sterile. Difference in cytoplasmic background of the individuals did nut influence their response to temperature. It is argued that partial male sterility is a normal feature of gynodioecious species and not an abnormality.