An efficient and simple method for constructing a genomic DNA library is presented by use of a TA cloning vector. It is based on sonicative cleavage of genomic DNA and modification of the fragment ends with Tag DNA polymerase, followed by ligation with a TA vector. This method was successfully applied to cloning of the phytoene synthase gene crtB from Spirulina platensis. The method is useful when the genomic DNA is not well digested with restriction enzymes owing to methylation or other reasons.