Fifty samples each of two leadmg brands of French, non-carbonated ('still') mineral waters, packed an plastic bottles, were stored in the dark for one month at approx~mateb. 200(. . to allot marked proliferation of their autotrophic microbial flora. Upon completion of th~s challenge test 1250 ml per bottle were filtered through five membranes. ~.h~ch were subsequently cultured on 1/10 strength nutrient agar for 48 h at a temperature favouring the growth of thermotrophtc orgam~ms only,, i.e 42 _+ 0 5°C. The numbers of colonies per one htre were below 10 ~ in all samples and did not exceed 200 in 56-80~. depending on the brand. Identification of the ~solates demonstrated strong inhibition of the psychrotrophic Gram-negative types, predominating m the association flora of stored stdl v, ater~ and preponderance of Gram-positive. catalase-posmve, facultatively anaerobic cfx:ci. Consequentl). a Reference Value for the thermotrophic autotrophic colony count per I ml m stdl, commercial mineral taters of the order m = I and M -5 seems justified. It is emphasized that. besides exammmg for these trl~.ial orgamsms, the usual tests for marker bacteria (Escherwht, coh and Lancefield group D .,treptoctv,,'cO and Pseud,monas aerugmosa should always be carried out