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The diversity and seasonal fluctuations of plant-parasitic nematodes (tylenchids) were investigated in five alfalfa fields in five counties of the Kerman Province of Iran during four consecutive seasons in 2013. Hundred soil samples were obtained per county, with 25 sub-samples per field being composited to five to represent five replicates per field per county per season. In total, 500 samples were analyzed during the study. Nematodes were extracted from soil samples using the modified tray method. In total, 11 plant-parasitic nematode genera and 12 species were recorded. According to prominence values, Ditylenchus (e.g. D. acutus, D. myceliophagus, D. terricolus and D. sarvarae), followed by Helicotylenchus pseudorobustsus, Pratylenchus neglectus and Meloidogyne javanica were the most prominent species. The season and the localities significantly affect the population densities of nematodes. Values for both the Shannon (H’) and Evenness (E) indices were the highest in Bam and the lowest in Jiroft counties. A significant, negative correlation existed between soil pH and mean population densities of Scutylenchus rugosus, while significant and positive correlations existed between soil electrical conductivity and Helicotylenchus pseudorobustus, Aphelenchoides sp., Amplimerlinius globigerus and Pratylenchus neglectus. In conclusion, diversity of plant-parasitic nematodes in Bam county was higher than other localities.