A full dust devil "season" was observed from Spirit from 10 March 2005 (sol 421, first active dust devil observed) to 12 December 2005 (sol 691, last dust devil seen); this corresponds to the period Ls 173.2¿ to 339.5¿, or the southern spring and summer on Mars. Thermal Emission Spectrometer data suggest a correlation between high surface temperatures and a positive thermal gradient with active dust devils in Gusev and that Spirit landed in the waning stages of a dust devil season as temperatures decreased. 533 active dust devils were observed, enabling new characterizations; they ranged in diameter from 2 to 276 m, with most in the range of 10--20 m in diameter, and occurred from about 0930 to 1630 hours local true solar time (with the maximum forming around 1300 hours) and a peak occurrence in southern late spring (Ls ~ 250¿). Horizontal speeds of the dust devils ranged from <1 to 21 m/s, while vertical wind speeds within the dust devils ranged from 0.2 to 8.8 m/s. These data, when combined with estimates of the dust content within the dust devils, yield dust fluxes of 3.95 ¿ 10-9 to 4.59-4 kg/m2/s. Analysis of the dust devil frequency distribution over the inferred dust devil zone within Gusev crater yields ~50 active dust devils/km2/sol, suggesting a dust loading into the atmosphere of ~19 kg/km2/sol. This value is less than one tenth the estimates by Cantor et al. (2001) for regional dust storms on Mars.