4-Hydroxyimino-oxazolidin-2-ones (3) were prepared by hydroxylaminolysis of 4-alkoxy-3-oxazolin-2-ones (2). Treatment of 3a with ethyl chloroformate in a molar ratio of 1:1 afforded 6, whereas the reaction 3a with two equivalents of ethyl chloroformate produced 4 and 5. By reacting 3a,d with diphosgene or thiophosgene the novel tetrahydro-oxazolo[ 4,3-c]1 ,2,4-oxadiazoles 7 could beobtained in low yields. From the prepared novel compounds only 3a displayed a remarkable fungicidal activity at 2 ppm.