The coat protein (CP) genes of the genomic RNA of 2 severe Turkish isolates of Zucchini yellow mosaic virus (ZYMV) from squash and muskmelon [ZYMV-Adana (Ad) and ZYMV-Ahlat (Ah), respectively] were cloned, and their complete nucleotide sequences and deduced amino acids were determined. The analysis revealed that both Turkish ZYMV-CP genes contained 837 nucleotides encoded for a CP of about 31.2 kDa. Phylogenetic trees based on nucleic acid sequences were constructed by the neighbor joining and unweighted pair group mean arithmetic (UPGMA) methods with 100 bootstrap replicates. A high degree of homology was detected between the 2 Turkish isolates on the nucleotide sequences (97%). The CP sequence of ZYMV-Ad and ZYMV-Ah varied among the 23 isolates with overall identity of 93%-98% and 94%-99%, respectively, at the nucleotide level. Comparison of the nucleotide sequences of 23 isolates from different geographical regions worldwide showed the ZYMV-Ad isolate clustered with isolates from Middle Eastern countries (Israel, Jordan, and Syria); ZYMV-Ah isolate was clustered with isolates from Far Eastern countries (Korea and Taiwan). The N-terminal of the Turkish ZYMV-Ah CP contained a distinctive sequence at nucleotide positions 9324-9328, which distinguished the Turkish ZYMV-Ah isolate from all previously reported ZYMV isolates. The CP cistron of the ZYMV-Ad and Ah isolates contained 279 amino acid residues. Pairwise nucleotide sequence comparison revealed sequence similarities of 58%-70% between ZYMV Turkish isolates and 22 other potyviruses. Mechanical inoculations showed that ZYMV-Ah produced faster systemic symptom induction than ZYMV-Ad on squash, suggesting that ZYMV-Ah was a more severe isolate than ZYMV-Ad (GenBank accession JF317296-JF317297).