To allow a direct comparison of the genomic DNA sequences of sufficiently similar organisms, there is an urgent need for software tools that can align more than two genomic sequences. We developed new algorithms and a software tool 'Multiple Genome Aligner' (MGA for short) that efficiently computes multiple genome alignments of large, closely related DNA sequences. For example, it can align 85% percent of the complete genomes of six human adenoviruses (average length 35305 bp.) in 159 seconds. An alignment of 74% of the complete genomes of three of strains of E. coli (lengths: 5528445; 5498450; 4639221 approximately bp.) is produced in 30 minutes.