RIGID ryegrass (Lolium rigidum) and sterile oat (Avena sterilis) are well known grasses in many parts of the world. They grow in both grasslands and arable habitats and both are recorded as serious weeds of arable lands in many countries worldwide. Seed germination of both species was tested against four levels of water salinity (0, 100, 300, 500mM/L of NaCl) and five different temperature regimes, constant at 8°C or alternating temperatures at 6/14, 8/16, 10/18 and 10/20°C, all in dark/light photoperiods of 8/16h, respectively. A. sterilis seeds generally showed higher germination rates than L. rigidum in all tested temperature regimes. Germination was very high in both species at alternating 10/20°C, and in A. sterilis also at 10/18°C, with nearly 40% in both. A. sterilis seeds also showed higher germination than L. rigidum at a constant temperature of 8°C, the latter favoring alternating temperatures. A. sterilis seeds resisted water salinity as high as 100 mM/L of NaCl and germinated at about 25%, compared to L. rigidum seed, which, while also showing some resistance to 100mM/L of NaCl only germinated at a rate of about 20%. Both species gave poor germination at higher levels of water salinity of 300 and 500mM/L of NaCl. Results confirmed that germination of seeds of both species favoured moderate alternating temperatures and showed some resistance to water salinity, which may indicate how both species can invade and persist in arable lands especially in Mediterranean climates and subtropics conditions.