Siliceous spicules of the freshwater Baikal sponge Lubomirskia baicalensis contain several proteins including silicateins. Existences of four different genes of silicatein alpha (alpha1, alpha2, alpha3, alpha4) which are related to silicatein alpha from the sea sponges were found when cDNA library analysis was made. The intron-exon structure of the full-size silicatein alpha1 gene was determined. This gene has total length of 1988 bp and includes 6 introns (1007 bp) and 7 exons (981 bp). With use of mass-spectrometric analysis of the spicule proteins tryptic digest, two silicateins alpha were authentically found.