Four A1 field isolates and one A2 field isolate of Phytophthora infestans were crossed to produce oospores in tomato leaves. The oospores were extracted and mixed with perlite and water, and healthy tomato leaves were used as bait for oospore-progeny infection. Twenty-nine lesions were obtained from the four crosses and 283 single-sporangium isolates were recovered and tested on four tomato differential lines carrying different major genes (Ph-0, Ph-1, Ph-2, and 3707) for late blight resistance. The pathogenic fitness (number of sporangia per unit leaf area) of parental and progeny isolates was strongly dependent on the host genotype; it decreased in the order Ph-0 > Ph-1 > Ph-2 > 3707. The A2 parent had a higher pathogenic fitness than the A1 parents on Ph-0 and Ph-1 but similar, lower fitness on Ph-2. Different levels of pathogenic fitness were observed across all isolates on Ph-0, although Ph-0 lacks resistance genes. Pathogenic fitness on one tomato genotype was not related to the pathogenic fitness on the other tomato genotypes. Some isolates exhibited reduced pathogenic fitness relative to the respective A1 parent, whereas others demonstrated a higher pathogenic fitness compared with the A2 parent. The tomato genotype Solanum pimpinellifolium L3707/5 was resistant to all five parental isolates of P. infestans. However, 37 of the 283 progeny isolates from 11 different lesions had compatible reactions with this line, producing up to 31 × 103 sporangia/cm2. Overall, reduced fitness was more frequent among the progeny isolates than increased fitness.