Pollen tubes formed following compatible and incompatible intraspecific matings in Petunia have been examined with light and electron microscopes. Compatible and incompatible tubes develop in an identical fashion on the stigma but, on entry into the top 1 mm of the stylar transmitting tissue changes occur both to the cytology of the tubes and their rates of growth. The early cytological changes are common to tubes of both compatibilities but, although both types of tube accelerate on entry into the style, incompatible tubes grow more slowly than compatible. Cytological differences became apparent between compatible and incompatible tubes following a short period of growth in the style, the latter possessing thicker cell walls and a cytoplasm packed with both organelles and reserves. Incompatible tubes subsequently burst or simply cease growth and die. The characteristic image afforded by this cytoplasm resembles that or burst or dead compatible tubes, except in that proportions of the cell components may differ. These data are discussed in terms of current models proposed to explain pollen tube growth and the operation of the self-incompatibility response in Petunia.