Abstract: A preliminary infection experiment was carried out during 2005-06 to identify slow yellow rusting sources of resistance in 20 NIA-wheat breeding lines along with Morocco as susceptible check at Nuclear Institute for Food and Agriculture, Peshawar. It was revealed that the current pathotypes of Puccinia striiformis West. tritici (Pst) were found virulent on all tested genotypes except NIA-6 and NIA-14. Host-pathogen compatibility in remaining lines facilitated the assessment of slow rusting trait and it was assessed through the epidemiological parameters viz., Final Rust Severity (FRS), Area Under Rust Progress Curve (AURPC) and Infection Rate (IR). Maximum FRS of 70% was rated in five lines while in the remaining 13 lines it ranged between 10 to 60%. Relative AURPC values up to 30% of the susceptible check were recorded for 10 lines while up to 70% for eight lines, showing desirable and good slow rusting behavior, respectively. Relative infection rate values ranged from 0 for NIA-7 to 1100-1300, recorded for eight lines. NIA-7 has a constant severity of 10% over assessment dates with lower AURPC value and was considered as better slow rusting line. Among various epidemiological parameters used for assessment of slow rusting trait, AURC was found to be a reliable parameter. Infection rate, on the other hand did not reveal the actual slow rusting behavior of the tested lines. Based on slow rusting trait, the tested lines may probably have genes for varying degrees of slow yellow rusting and can be used for further manipulation in wheat improvement program after confirmatory studies.