Video microscopy was used to examine a genetically tagged mutant (T30) of Glomerella graminicola whose conidia have a propensity to burst during the germination process. Before the germ tube is produced, the cell wall ruptures and the cytoplasm is extruded. The bursting takes place in less than 0.1 second. Bursting can be prevented by adding osmotica to the germination medium. By phase-contrast microscopy of alkali-washed conidia, we found that each ruptured conidium had a single gaping hole that was oriented parallel to the long axis of the spore. The holes were more frequent in the middle region of the conidia than at the apices. A mathematical model for stress based on conidial shape indicated that conidia ruptured where the stress on the wall was greatest. The microscopic observations on the orientation and distribution of the rupture sites are consistent with the hypothesis that T30 conidia have weaker walls than the wild-type. Comparative tests of resistance to mechanical breakage support this hypothesis.