The European chestnut (Castanea sativa) is threatened by the hemibiotrophic oomycete Phytophthora cinnamomi, the causal agent of ink disease. Chestnut species have different susceptibility levels to P. cinnamomi, with the Asian species (C. crenata; C. mollissima) exhibiting the highest level of resistance. A histological approach was used to study the responses exhibited by susceptible and resistant chestnut genotypes by characterizing the early stages of P. cinnamomi infection and the cellular responses it induces in roots. C. sativa (susceptible) and C. crenata (resistant) plantlets, were inoculated with a P. cinnamomi virulent isolate with a zoospore suspension or by direct contact with mycelia agar plugs. Root samples were collected at 0.5, 3.5, 24, 48, and 72 hours after inoculation (hai) for microscopic observations. Penetration was observed, in both species, at 0.5hai and 3.5hai with mycelium and zoospore inoculations, respectively. In both inoculation methods, following the penetration into the rhizodermis, P. cinnamomi hyphae grew inter- and intracellularly through the cortex and into the vascular cylinder. C. crenata cells displayed a delay in the pattern of infection, by having fewer cell layers colonized when compared with C. sativa. At 72hai, the collapse of the first layers of C. sativa cortical cells was observed, indicating the beginning of necrotrophy. C. crenata was able to respond more efficiently to P. cinnamomi than C. sativa, by restricting the pathogen’s growth area through the early activation of resistance responses, such as callose deposition around some intracellular hyphae, HR-like cell death, cell wall thickening and accumulation of phenolic-like compounds.