The level of contamination with organochlorine pesticides and the occurrence of their degradation products in the basins of the two main rivers, San Juan and Jáchal, of the Province of San Juan, Argentina, were determined. Surface and ground water samples from both river basins were evaluated by capillary GC and results confirmed with Mass Spectrometry. Chemicals investigated were: alfa-hexachlorocyclohexane (alpha-HCH); beta-hexachlorocyclohexane (beta-HCH); gamma-hexachlorocyclohexane (gamma-HCH); delta-hexachlorocyclohexane (delta-HCH); Heptachlor; Heptachlor epoxide; Aldrin; Dieldrin; Endosulfan I; Endosulfan II; 1,1,1-trichloro-2,2 bis (p-chlorophenyl) ethane (p,p´ DDT; 1,1-dichloro-2,2 bis (p-chlorophenyl) ethylene (p-'DDE); 1,1-dichloro-2,2-bis (p,p'-chlorophenyl) ethane (p-p'DDD); 1,1-dichloro-2,2-bis-(o,p'-chlorophenyl) ethane (o-p'DDD); Methoxychlor; Endrin. Concentrations found for beta-HCH and Dieldrin were above the maximum permissible values recommended by international organizations (E.C. Council Directive 1980). Mean values found in positive samples were 14.20 mug/L (0.01 - 208.10) and 9.35 mug/L (0.07 - 145.00) for b-HCH and Dieldrin, respectively. For a total number of 314 samples, the percentage of positive samples ranged from 68.6 % for Heptachlor to 16 % for Aldrin. Concentration values and the percentage of positive samples in ground water were significantly lower than those found in surface water. In some surface samples, the values were higher than the allowed maximum (1 to 3 mug/L). Samples taken in different seasons did not show significant differences