Helium isotope measurements in six major basins in the Gulf of California show that the deep Guaymas Basin has 3He/4He 65--70% Higher than atmospheric helium, clear evidence of mantle helium injection. Smaller 3He excesses observed in the Carmen and Farallon basins may be derived from this Guaymas Basin anomaly. The 3He concentrations in the Mazatlan Basin in the mouth of the Gulf of California are similar to average eastern Pacific values, indicating that the Gulf does not provede a significant flux of 3He into the general Pacific circulation. On the basis of temperature and salinity measuremens an upper limit of 0.28 ¿C can be placed on the amount of geothermal heating observed in any of the basins. The isotopic ratio of the injected Guaymas Basin helium is found to be 3He/4He = (1.10¿0.06) ¿10-5, almost identical to the heliun signature observed at the Galapagos Rift but somewhat lower than the average ratio in oceanic basalt glasses.