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Analyses of closed-isobar, non-migratory, thermal low-pressure frequencies are presented for an 11-year period, 1974-1984, over the south-western USA and north-western Mexico. Average onset and breakdown dates, preferred locations, and the vertical extent of the system are presented. The effect of upper level (500 mbar) flow on the presence of the thermal low and relationships between the system and precipitation in Arizona and western Mexico also arc investigated. The analyses suggest a preferred region for the formation and sustenance of the thermal low near the south-west coast of the USA and the west coast of Mexico. Within this region exist two frequency maxima, centred over the desert regions of the American South-west and north-western Mexico and over the plateau highlands of the Sierra Madre Occidental. The system is most prevalent from mid-June until mid-September and reaches its maximum vertical "tent near 700 mbar in July and early August. This variation may be attributed to the zenithal progression of the sun, although short-term fluctuations in the strength of the system during the summer period probably are related to changes in cloud cover over the low. Specifically, precipitation in advance of 500-mbar troughs passing to the north of the system appears to be associated with the weakening of the thermal low, suggesting a negative feedback relationship that deserves further investigation.