As a first step towards understanding the physiological role and regulation of the expansin gene (EXP) family in Coffea arabica fruits during growth and maturation, we identified 11 expansin genes, nine belonging to the a-expansin family (EXPA), one EXLA and one EXLB, through in silico analysis of expressed sequence tags (ESTs). Within the a-expansin family, three isoforms were selected for detailed examination based on their high expression in coffee fruits or because they were specifically induced during different fruit developmental stages, according to the EST information. The expression patterns were analysed in different fruit tissues (perisperm, endosperm and pericarp) of C. arabica cv. IAPAR-59 and C. arabica cv. IAPAR-59 Grau´ do, the latter being a closely related cultivar with a larger fruit size. Accumulation of CaEXPA1 and CaEXPA3 transcripts was high in the perisperm (tissue responsible for coffee bean size) and in the early stages of pericarp development. Transcripts of CaEXPA2 were detected only in the pericarp during the later stages of fruit maturation and ripening. There was no detectable transcription of the three EXPs analysed in the endosperm. The observed differences in mRNA expression levels of CaEXPA1 and CaEXP3 in the perisperm of IAPAR-59 and IAPAR-59 Grau´do suggest the participation of these two isoforms in the regulation of grain size.