The genus Pelophylax has the most controversial taxonomy of any group of western Palearctic frogs. The taxonomic status of the Pelophylax ridibundus sensu lato from Iran is unclear and many populations of this marsh frog have not been examined. Here, we investigate cytogenetic and morphological variation in populations of this species from western and southwestern Iran. The morphometric results revealed that the frogs from these two areas can be largely distinguished in discriminate function analyses. All frogs are diploid and have 2N=26 chromosomes with five large pairs and eight small pairs of chromosomes. These chromosomes are metacentric, submetacentric and subtelocentric. All frogs have three or four pairs of subtelocentric chromosomes. The 6^th and 13^th pairs of chromosomes are subtelocentric in all western populations, but the position of the third subtelocentric chromosome pair varies. In south-western populations (Bushehr and Deylam regions) the 8^th, 9^th, and 13^th >pairs of chromosomes are subtelocentric, but frogs from Bushehr have one more pair of subtelocentric chromosomes (11^th). The second pair of large chromosomes in western populations is submetacentric, but in southwestern populations, the second pair of large chromosomes is metacentric. These differences in the type of chromosomes of P. ridibundus, especially in the presence, absence and number of subtelocentric chomosomes, suggest that the taxonomy of Pelophylax in Iran requires further investigation.