The ER body is a novel compartment that is derived from endoplasmic reticulum (ER) in Arabidopsis. In contrast to whole seedlings which have a wide distribution of the ER bodies, rosette leaves have no ER bodies. Recently, we reported that wound stress induces the formation of many ER bodies in rosette leaves, suggesting that the ER body plays a role in the defense system of plants. ER bodies were visualized in transgenic plants (GFP-h) expressing green fluorescent protein (GFP) with an ER-retention signal, HDEL. These were concentrated in a 1000-g pellet (P1) of GFP-h plants. We isolated an Arabidopsis mutant, nai1, in which fluorescent ER bodies were hardly detected in whole plants. We found that a 65-kDa protein was specifically accumulated in the P1 fraction of GFP-h plants, but not in the P1 fraction of nai1 plants. N-terminal peptide sequencing revealed that the 65-kDa protein was a β-glucosidase, PYK10, with an ER-retention signal, KDEL. Immunocytochemistry showed that PYK10 was localized in the ER bodies. Compared with the accumulation of GFP-HDEL, which was associated with both cisternal ER and ER bodies, the accumulation of PYK10 was much more specific to ER bodies. PYK10 was one of the major proteins in cotyledons, hypocotyls and roots of Arabidopsis seedlings, while PYK10 was not detected in rosette leaves that have no ER bodies. These findings indicated that PYK10 is the main component of ER bodies. It is possible that PYK10 produces defense compounds when plants are damaged by insects or wounding.