The simple nervous system and complete genomic sequence of C. elegans has made it a powerful system for genome wide approaches to behavior. Although there are only 32 chemosensory neurons of 14 types in C. elegans, preliminary results indicate that there are ~700 olfactory receptor genes, number resembling these for other organisms including mouse, rat and human. More recently, sequencing of an estimated 98% of the C. briggsae genome (manuscript in preparation) provides an ideal platform for carrying out comprehensive comparative genomic analysis between C. briggsae and C. elegans genomes. In this study, we examined the olfactory subgenomes of these two closely related organisms. A similar number (~500) of olfactory genes have been identified for C. briggsae. Of all these genes, there are >330 orthologs. C. elegans olfactory subgenome has ~200 more genes than that of C. briggsae. Interestingly, these "extra" genes all fall in to two of a total six Pfam olfactory subfamilies, making the number of genes for C. elegans in these two subfamilies twice that of C. briggsae. The numbers for olfactory genes in the other four subfamilies for both organisms, however, remain similar. Detailed comparative analysis of chromosomal localization of these orthologous and unique olfactory genes will help us understand the dynamic nature of these genes and how these cousins adapt to their own environment by evolving their own set of olfactory genes to accommodate their abilities to feed, mate and communicate efficiently.