Abstract Deep lagooning is an alternative to conventional lagooning and it implies smaller land requirements as an additional advantage. Treatment in deep wastewater stabilization ponds is influenced by thermal stratification. The object of this paper is the study, over a 1-yr period (October 1986–December 1987), of the influence of temperature on the behaviour of a treatment unit (an 8 m deep pond) which receives domestic wastewater. For this study, a spatiotemporal follow-up of the natural treatment process has been conducted. The removal efficiencies achieved for the following parameters varied as follows: COD, 71–92%; BOD 5, 83–97%; total Kjeldahl nitrogen, 51–98.8%; ammonia nitrogen, 38.8–99.9%; total phosphorus, 42.2–92.9% and orthophosphate, 40–99%. Log reduction of microorganisms varied between 2.0 and 3.0 for total coliforms, 1.1–3.0 for faecal coliforms and 1.7–3.3 for faecal streptococcus. Other physicochemical parameters observed were: temperature, dissolved oxygen, pH, nitrates and nitrites. The continuous flow of wastewater did not hinder the appearance of a marked stratification in spring-summer: an aerobic epilimnetic zone and an anaerobic hypolimnetic one remained clearly differentiated. The evolution of all the parameters observed was similar. In the absence of thermal stratification, the system was found to function in approximately complete mixing conditions, and the value of each parameter was almost constant throughout the water column; nevertheless, the presence of stratification caused a vertical distribution with marked gradients in the thermocline zone.