The variability of 12 Echinochloa populations with respect to certain morphological and physiological characteristics, sensitivity to certain POST rice herbicides, and activity of selected enzymes was studied. Three distinct groups (each one represented by four populations from different areas) were studied: E. crus-galli, E. oryzoides, and E. phyllopogon. All the E. oryzoides and E. phyllopogon populations showed earlier seed germination and higher germination percentages than the E. crus-galli populations. All the E. oryzoides and E. phyllopogon populations showed reduced susceptibility to propanil, cyhalofop, clefoxydim, and bispyribac compared with the E. crus-galli populations. With respect to plant prostrateness, the species order, averaged over the four populations, was E. crus-galli prostrate > E. oryzoides relatively erect > Ε;. phyllopogon erect, while the species order with respect to leaf length and tillering ability was E. crus-galli > E. oryzoides > Ε;. phyllopogon. Regarding leaf width, time of panicle emergence, height, and biomass accumulation, the order was E. crus-galli > E. oryzoides > Ε;. phyllopogon, while that of seed weight, length and width was E. oryzoides > Ε;. phyllopogon > E. crus-galli. The order of species susceptibility (averaged over the four populations) to most of the herbicide treatments was E. crus-galli > E. oryzoides > E. phyllopogon, which was exactly the opposite of that relating to their antioxidant enzyme activity. Finally, the order of herbicide efficacy, averaged over all Echinochloa populations, was penoxsulam > clefoxydim > bispyribac > cyhalofop > propanil. Variability in a number of traits among the most common Echinochloa species of rice fields in northern Greece as a result of different adaptive strategies of each species may be related to differential sensitivity to herbicides. This variability should be taken into account for the elaboration of effective weed management programs in rice. Where mixed populations of these species are present in a field, difficulties may arise in the successful chemical control of the Echinochloa complex in rice due to species differences in biology and herbicide sensitivity.