We identified 277 alternative splice forms in silkworm genes based on aligning expressed sequence tags with genomic sequences, using a transcipt assembly program. A large fraction (74%) of these alternative splices are located in protein-coding regions and alter protein products, whereas only 26% are in untranslated regions. From the alternative splices located in protein-coding regions, some (43%) affect protein domains that bind various biological molecules. The vast majority of the detected alternative forms in this study appear to be novel, and potentially affect biologically meaningful control of function in silkworm genes. Our results indicate that alternative splicing in silkworm largely produces protein diversity and functional diversity, and is a widely used mechanism for regulating gene expression.