Infection biology and defence responses of watermelon attacked by Stagonosporopsis cucur-bitacearum (syn. Didymella bryoniae) were studied in two genotypes, accessions PI189225 (moderatelyresistant) and 232-0125/B (susceptible). On intact leaf surfaces, spores started to germinate 14 hafter inoculation (hai) with one to three germ tubes, which subsequently developed and formedappressoria. Invasion of the host tissue started at 20 hai by direct penetration from appressoria oroccasionally indirectly through stomata. In the susceptible accession, a significantly higher numberof direct penetrations were observed than in the moderately resistant. After invasion, hyphal coloni-sation was restricted in the intercellular spaces in the moderately resistant accession, whereas theydeveloped extensively, causing tissue decay, in the susceptible accession. Macroscopic symptomswere seen in leaves of the moderately resistant accession as small and dry lesions, whereas big,water-soaked lesions developed on the susceptible accession within 48 hai. Investigations of thedefence responses of the two accessions showed accumulation of H2O2 at penetration sites beneathappressoria in the moderately resistant, but to a lesser extent in the susceptible accession. SuchH2O2 accumulation correlated with a reduction in penetration frequency and a lower level of hyphalgrowth after infection in the moderately resistant accession. There was a rapid and early increasein total peroxidase as well as β-1,3-glucanase activity in the moderately resistant compared to thesusceptible accession. These results indicate that fungal penetration and development in watermelonare inhibited by a consorted action of different responses including accumulation of H2O2, peroxidaseand β-1,3-glucanase.