SUMMARY The effect of heat on potato cyst eelworm cysts, Heterodera pallida Stone (British pathotype E) and H. rostochiensis Woll. (British pathotype A) was investigated in the laboratory in 1967–69. Hatching from wet cysts of H. pallida was completely inhibited after 4 minutes’ exposure to 50°C and above. Air‐dry cysts were more resistant to heat; hatching was only prevented at 90°C. Results indicate that H. pallida is more susceptible to heat than H. rostochiensis.