Xenorhabdus nematophila are Gram-negative bacteria that engage in mutualistic associations with entomopathogenic nematodes. To reproduce， the nematodes invade insects and release X. nematophila into the haemolymph where it functions as an insect pathogen. In complex medium， X. nematophila cells produce two distinct types of intracellular crystalline inclusions， one composed of the methionine-rich PixA protein and the other composed of the PixB protein. Here we show that PixB crystalline inclusions were neither apparent in X. nematophila cells grown in medium that mimics insect haemolymph (Grace's medium) nor in cells grown directly in the insect haemocoel. The identified pixB gene was regulated by a conserved σ70 promoter while the pixA promoter was less well conserved. Expression of pixA and pixB under biological conditions was analysed using GFP promoter reporters. Microplate fluorescence detection and flow cytometry analyses revealed that pixB was expressed at high levels in Grace's medium and in insect haemolymph and at lower levels in complex medium， while pixA was expressed at lower levels under all conditions. Although pixB was highly expressed in Grace's medium， PixB crystalline inclusions were not present， suggesting that under biological conditions PixB production may be controlled post-transcriptionally. Although a pixB-minus strain was constructed， the function of PixB remains unresolved. The pixB gene was present in few Xenorhabdus species and pixB-type genes were identified in some Proteobacteria and Gram-positive species， while pixA was only present in Xenorhabdus species. Two conserved sequences were identified in PixB-type proteins that characterize this previously unrecognized gene family.