In 2004, the organophosphorus pesticide, diazinon, was phased-out for all residential uses in the United States. The objective of this study was to determine the temporal trend of diazinon levels in the Chollas Creek, CA watershed since the phase-out of this pesticide. Stormwater samples from Chollas Creek were collected during seven storm events in 2006–2007. The median diazinon level for all samples was 0.13 mg/L. Statistical analysis using the Kruskal-Wallis test revealed no statistically significant (p50.765) spatial difference among any of the sampling sites. Correlational analysis (using the Spearman’s rho test) revealed that there was no significant association between antecedent dry days and median diazinon levels (p50.383, rho50.393). Additionally, no association between storm event precipitation (p50.355, rho520.414), median storm event intensity (p50.585, rho520.252), or annual cumulative precipitation (p50.760, rho50.143) was observed. Trend analysis of diazinon levels (1998–2005) showed that levels in this watershed have declined with a negative slope of 0.0002 mg/L per year, and statistical analysis (using the Mann-Whitney U test) showed there was a significant decline in levels after the pesticide’s phase-out in 2004 (p,0.05).