Harris County Mosquito Control Division (HCMCD) is responsible for surveillance of mosquito species that are vectors of St. Louis Encephalitis (SLE) virus and West Nile Virus (WNV) within Harris County, Texas, including the Houston metroplex. The metroplex area has some unique attributes and a vast variety of environmental habitats that are attractive to vectors of arboviruses and for the transmission of arboviruses to the human population. Data describing the efficacy of Gravid (GV) and Underhouse (UH) CO2 traps were analyzed to determine if there is a significant difference between these two trap types with respect to the number of mosquitoes and the variety of mosquito species caught. This study was conducted during the off-peak HCMCD trapping season, to gain information in preparation for a yearround trapping program utilizing Underhouse CO2 traps for WNV and SLE virus surveillance. Adjusting for the week of collection, results suggest that Gravid traps caught significantly (P = 0.009) more mosquitoes (mean = 23.134 per trap) in the study area than Underhouse traps (mean = 3.616 per trap), and that Underhouse Traps caught a larger variety of mosquito species (n = 13) than Gravid Traps (n = 11), out of 15 total different species caught. Gravid and Underhouse traps caught 9 out of 15 of the same mosquito species during the study period. Culex quinquefasciatus mosquito catches in Gravid traps and temperature were strongly correlated (Spearman's Correlation Coefficient = 0.707, P = 0.005). Geographic Information System spatial analysis indicated clustering of Culex quinquefasciatus mosquito catches in both Gravid traps, week 9 and 21 (Moran's I = 0.69, P = 0.040 and 0.74, P = 0.021, respectfully ) and Underhouse traps, week 13 and 19 (Moran's I = 0.92, P = 0.002, and 0.89, P = 0.011, respectfully). It is recommended that Harris County Mosquito Control Division continue to utilize gravid traps as a primary method of surveillance. Gravid traps (16,194) caught 85% more mosquitoes than Underhouse traps (2,531) over the fourteen week study period. Their overall success far outweighs the additional materials or labor required for their use in a successful surveillance program.