Unique cyclic carbamate structures of new plant growth regulators, cyclocarbamides A and B, from an unidentified Streptoverticillium sp. were established via X-ray crystallographic analysis and spectrometrical methods. In a screening for microbial-origin plant growth regulators, the broth filtrate of this Streptoverticillium sp. inhibited lettuce seed germination. Active components—new cyclic carbamate derivatives cyclocarbamides A and B, plus streptimidon—were isolated using solvent extraction, silica gel column chromatography, and preparative HPLC. Cyclocarbamide A (mp 109–110°C, [α]D -259°) had a molecular formula C14H20N2O4 (confirmed by EIMS, FABMS, and 13C-NMR), with its structure conclusively determined by X-ray crystallography (orthorhombic, P212121 space group, a=14.329(2) Å, b=14.880(3) Å, c=6.786(2) Å, Z=4; R=0.053 for 1403 reflections). Cyclocarbamide B (C15H22N2O4) was a homolog of A, containing a hexanoyl moiety instead of A’s 3-methyl butanoyl group. Both compounds inhibited lettuce seed germination at 30 ppm but showed no seedling growth inhibition at 100 ppm. The cyclic carbamate structure explains their alkali lability, and such structures are rare among microbial metabolites.