The thermal behaviour of some ethylene-propylene random copolymers (EPRs) with 55-70 mol% ethylene content was investigated in the temperature range from -100 to +200 degrees C. Irrespective of differences in chemical composition broadness, resulting from different catalytic systems, all samples display a low-melting crystallinity of low degree, developing rapidly when the material is cooled to low temperatures. The poorly crystalline phase, undetectable by X-rays, melts over a broad range of temperature, the lower limit merging with the glass transition and the upper one ending at similar to 60 degrees C. The crystal size distribution is affected by isothermal treatments performed below room temperature, but the overall degree of crystallinity is not modified. The low-melting crystalline domains are associated with short ethylene segments comprising < 10 CH2 sequences. Experimental evidence suggests that 55 mol% is the lower limit of ethylene content in EPR copolymers required to achieve a detectable crystallinity which melts below room temperature.