A gene (At4g20010) involved in regulating flowering time in Arabidopsis thaliana (L.) Heynh. was identified by promoter trap T-DNA tagging. Plants containing a T-DNA insert in the 3′-UTR of At4g20010 flowered later under both long- and short-day conditions compared with control plants. Histochemical assays of the mutant plants showed that the promoterless gus gene is expressed predominantly in the shoot apex, but it is also expressed in root tips, stem nodes and in the abscission zone of developing siliques. Measurement of endogenous gibberellin (GA) showed that bioactive GA₄ levels in mutant plants were reduced compared with wild type (WT) plants. Like other known mutants defective in GA biosynthesis, the late-flowering phenotype observed in our T-DNA-tagged line could be largely repressed by application of exogenous GA₃. The T-DNA-tagged gene At4g20010 encodes a previously uncharacterised protein belonging to the DUF731 family. Sequence analysis showed similarity to a single-stranded binding domain and to an RNA-binding protein of Chlamydomonas reinhardtii. Considering the above results (sequence similarity, mutant phenotype and level of endogenous GA), we propose that At4g20010 is an RNA-binding protein involved in regulating GA biosynthesis, possibly at the post-transcriptional level.