A new stablegermaphosphene, the 2,2-dimesityl-1-(2,4,6-triipropylphenyl)germaphosphene (1), has been made. It reacts quantitatively with benzaldehyde and α-phenyl-N-tert-butylnitrone to afford, by [2+2]- and [2+3]-cycoloadditions, respectively, the correponding four- and five-membered heterocycles 10 and 13. The conformations of these heterocycles are discussed. The reactions of 1 and 2,2-dimensityl-1-(2