The SPINDLY (SPY) locus of Arabidopsis thaliana is believed to be involved in gibberellin (GA) signal transduction. The six known mutations at this locus cause a phenotype that is consistent with constitutive activation of the GA signal transduction pathway. spy alleles are epistatic to gai, a mutation conferring gibberellin-insensitivity, indicating that SPY acts as a negative regulator of GA signal transduction, downstream of GAI. SPY was cloned using a T-DNA insertion in the spy-4 allele. SPY encodes a 914 amino acid protein with an N-terminal TPR region (a likely protein-protein interaction domain) and a novel C-terminal domain. The spy mutants show that both the N- and C-terminal domains of SPY are functionally important, spy-4 is likely to be a null allele and displays some morphological defects not seen in the other alleles. A 35S:SPY construct rescues the spy mutant phenotype, but does not show any gain-of-function SPY phenotypes. Smaller constructs overexpressing different domains of the SPY protein have no effect on plant development.