- The red imported fire ant Solenopsis invicta is the most destructive invading arthropod in the southern United States, yet little is known about its genome complexity and organization. Here we report the size, organization and GC content of S. invicta genome. DNA reassociation kinetics using S1 nuclease assay and a modified second-order kinetics model indicated that the S. invicta genome is approximately 0.62 picograms or 5.91 x 10(8) base pairs, composed of 36% unique, 41% moderately repetitive and 23% highly repetitive/foldback sequences. Comparison of the reassociation kinetics of short and long DNA fragments revealed that the sequence arrangement follows a pattern of short period interspersion, as in most organisms with relatively large genomes. Melting-temperature analysis showed that the GC content of the fire ant genomic DNA is 34.8%, similar to that of most eukaryotic organisms. The results reveal that the fire ant genome is much larger and more complex than those of a number of hymenopteran insects studied to date. Our study provides a foundation for further analysis and genetic manipulation of the S. invicta genome.