Aqueous dispersions of the anionic phospholipid dimyristoyl phosphatidylglycerol (DMPG) in 100 mM ionic strength, at pH values higher than 4, are known to exhibit a thermal behavior rather similar to that of the zwitterionic lipid dimyristoyl phosphatidylcholine (DMPG), which undergoes a gel-to-liquid crystalline phase transition at 23 degreesC. However, in aqueous medium of low ionic strength, DMPG was shown to present a large gel-fluid transition region, ranging from 18 to 35 degreesC. This intermediate phase is optically transparent and characterized by a continuous change in membrane packing. The present work shows that, below a certain lipid concentration, which we denote c' (c' = 0.4 +/- 0.2 mM), this anomalous behavior vanishes, being replaced by a sharper phase transition centered at a unique transition temperature T-m. Interestingly, T-m increases as the lipid concentration is further decreased, reaching a limiting value of about 41 degreesC. Below c', opposite to the effect observed for DMPG at higher concentrations, the value of T decreases as the ionic strength is increased. Above 500 mM NaCl, the thermal behavior of DMPG aqueous dispersions is roughly independent of the lipid concentration. Furthermore, increasing sample pH significantly decreases T-m in the low concentration regime, whereas in concentrated DMPG dispersions, T-m is not affected by changes in pH above 6. The dependence of the thermal behavior of dilute DMPG aqueous dispersions on pH and ionic strength is discussed here in the light of the dependence of the headgroup protonation on DMPG concentration.