A series of new acetohydrazone-containing 1,2,4-triazolo[1,5-a]pyrimidine derivatives were designed and synthesized for the purpose of searching for novel agrochemicals with higher fungicidal activity. Their in vitro fungicidal activities against Rhizoctonia solani were evaluated, and the most promising compound, 2-[(5,7-dimethyl[1,2,4]triazolo[1,5-a]pyrimidin-2-yl)sulfanyl]-2'-[(2-hydroxyphenyl)methylidene]acetohydrazide (2-17), showed a lower EC(50) value (5.34 microg ml(-1)) than that of commercial carbendazim (EC(50)=7.62 microg ml(-1)). Additionally, compound 2-17 was also found to display broad-spectrum fungicidal activities, and its EC(50) value (4.56 microg ml(-1)) against Botrytis cinereapers was very similar to that of carbendazim. Qualitative structure-activity relationships (QSARs) of the synthesized compounds were also discussed.