The red imported fire ant, Solenopsis invicta, has been collected at several Mexican locations along the US-Mexico border. Fire ant identification can be challenging. We sampled fire ants, Solenopsis spp., at selected locations of northeastern Mexico an examined their maternally-inherited mitochondrial citochrome oxidase gene. We detected previously reported markers of Solenopsis invicta and the native Solenopsis xyloni, and previously undescribed markers of the native Solenopsis geminata. We also detected the S. xyloni marker on specimens identified as S. geminata male x S. xyloni female hybrids, in agreement to the proposed parentage of the hybrid. Thus the fire ants of Northeastern Mexico include Solenopsis invicta, Solenopsis geminata, native ants harboring the Solenopsis xyloni marker, and a hybrid as evidenced from genetic markers and morphology. This hybrid seems to be prevalent at locations on the Gulf plains, and its large-headed workers could be misidentified as those of Solenopsis geminata. These findings have relevance in management, biodiversity and ecological studies.