This research aimed to evaluate the effect of the epigenetic climatic factor of heat stress expressed by the THI on some parameters of milk production of Prim'Holsteins cows kept in the climate of the Algerian West coast. The THI was used as an indicator of the heat stress threshold. A sample of 75 lactating cows was divided into three groups according to THI level. The cows of group 1, with severe stress (THI>78), those of group 2, with light stress (THI 72-78), and those of group 3 in thermal comfort (THI<72). The daily milk production (DMY), the Physico-chemical quality (TB, TP, and EST), and the hygienic quality (total flora FMAT) of raw milk showed a decrease with the increase of the THI threshold. A significant effect (p<0.05) of heat stress is recorded on the Physico-chemical and microbiological parameters of milk. The FMAT count values of raw milk samples were highest for groups 1 and 2, while a low bacterial load was recorded for group 3 (THI<72). Heat stress had a negative impact on the performance of Prim'Holsteins cows. These preliminary results show the severity of the negative impact of heat stress in the littoral region on the production and the physicochemical and hygienic quality of milk and will serve as a useful guide for farmers to adopt actions to mitigate the impact within the barns for the sustainability of livestock.