Based on the element maps of twenty-two gibberellin methyl esters fragmentations are discussed, which are characteristic of the common structural features as well as structural modification in gibberellin homologues.PREVIOUSLY twenty-four gibberellin~,~.~.~ gibberellins A, -AZ4, have been isolated from fungi and higher plants and their structures established. These compounds are conventionally classified into two groups, C,, and C,, gibberellins, according to the number of carbon atoms in the molecules. Both groups are common in containing the gibbane ring (1) which is substituted with a P-oriented methyl at C-1 (except A,, and A,,), an exomethylene or its equivalent at C-8 and a P-oriented carboxyl at C-10, but they differ from each other in the following points. C,, gibberellins contain an a-oriented y-lactone (C-1 -+ C-4a), whereas C,, gibberellins bear an a-oriented carboxyl at C-1 and a one-carbon substituent such as methyl, hydroxymethyl, aldehyde or carboxyl at C-4a. Introduction of varied substituents, i.e. hydroxyl, epoxide and double bond, into the basic skeleton constitutes a so-called gibberellin group with so many members. Therefore, systematic mass spectrometry for large numbers of gibberellin homologues is expected to afford fundamental information for identification and structural elucidation of new gibberellins. 9 8 (1) Wulfson et al. have reported the mass spectra of methyl esters of gibberellins A,, A, and A4,4 and further discussed fragment patterns depending on the position 71 1 ...R3