The main objective of this research was to characterize variability in the Cochliobolus sativus (anamorph Bipolaris sorokiniana)-barley-wheat pathosystem in Oman, as a model system for studying variability in host and pathogen populations. Several studies were conducted on variability, resistance, culture filtrate effects and cereal responses to black point infection, using eleven B. sorokiniana isolates, nine barley varieties and four wheat varieties. B. sorokiniana, Alternaria spp. and Fusarium spp. were isolated from barley seeds with mean frequencies of 4.7%, 6.2% and <0.25%, and from wheat seeds with mean frequencies of 1.4%, 11.5% and 0%, respectively. B. sorokiniana was also isolated from the shoot bases of barley (0-41.5%) and wheat (0-0.5%). A significant correlation (r=0.730, P<0.05) was found between seed and shoot base infection in barley. Wheat crowns were found to be significantly less susceptible to B. sorokiniana infection than barley crowns. B. sorokiniana and Alternaria spp. were isolated from barley seeds showing black point disease symptoms. The disease was found to affect seed germination and seedling length and dry weight. Severity of chlorosis was found to be the best indicator for quantifying variability in spot blotch resistance. The barley variety BA was found to be the most resistant, while J51 was the most susceptible. The wheat varieties K65 and WC were more resistant to spot blotch, while WM and WS were susceptible. Very limited isolatevariety interaction was found. The results for spot blotch resistance from the detached leaf method agreed with those from intact plants. Benzimidazole (60ppm) had no significant effect on growth rate and sporulation of B. sorokiniana in culture. No significant difference was observed in resistance to root rot among barley varieties or wheat varieties, but wheat varieties were found generally to be more resistant to root rot than barley varieties. No correlation was found between resistance to root rot, shoot base discoloration and leaf spot at P<0.05. The culture filtrate of B. sorokiniana induced necrotic-like tan symptoms associated with loss in green color in barley and wheat leaves. Different varieties of barley and wheat showed significant differences (P<0.05) in reaction to the culture filtrate of different isolates, but no correlation was found with spot blotch symptoms. Significant differences were observed between B. sorokiniana isolates in morphology of conidia, growth rate, germination at different temperatures, virulence on barley leaves and wheat leaves, virulence on barley roots, and culture filtrate (toxin) effect. The length, width and number of septa in the conidia varied from 45-145um, 12.5-35 um, and 4-13, with means of 90.5 um, 20.5 um and 8. No correlation was found between the morphology of conidia, growth rate, germination at different temperatures, virulence on leaves, virulence on roots and culture filtrate effect of different isolates. Principal component analysis was used to examine the degree of variability in B. sorokiniana isolates. The relatedness of isolates was best described using all the characteristics described previously.