In 2008 and 2009, our current thermal fogging methods and materials were evaluated in underground systems against feral and laboratory strains of adult Culex quinquefasciatus. Culex quinquefasciatus adults collected from storm drain systems the night before treatment were tested concurrently with adults from a susceptible laboratory strain (Sebring) in 10 separate tests. During 2008, there were no significant differences in the low mean percent control obtained between any of the feral populations (29.39%) and susceptible (Sebring) populations (56.04%) tested, whereas in 2009, application of mineral oil alone yielded extremely low, but significantly different mean percent control at 0.99% for ferals and 0.01% for Sebrings. During 2009 mineral oil evaluations, 45,677 droplets were collected in storm drains at distances of 99.1, 50.6, 57.9, 67.7, and 109.7 m from the application site, with 99% of the droplets below 3 µm in diam; additionally, we found no significant differences between mean percent control of Sebring and feral mosquito populations using the higher (3.2×) 1∶10 application rate of Pyrocide®. However, mean percent control between the feral and susceptible strain (Sebring) during 2009 was lower than in 2008 at 16.55% for ferals and 24.43% in Sebrings. Results indicated that control methodologies and/or chemicals used were ineffective at controlling Cx. quinquefasciatus in storm drains using the chosen experimental design. Based upon this information, thermal fog operations were discontinued due to lack of effectiveness.