The effect of Mycoplasma gallinarum isolated from village chickens on embryonated village chicken eggs was investigated. Seven-, 10-, 12- and 18-day-old embryos were inoculated with medium containing 10(6) colony forming units of M gallinarum or with uninfected medium and incubated at 37 degrees C until they hatched. There was no significant difference between the infected embryos which failed to hatch and those which hatched or between them and the group inoculated with medium only. A much higher percentage of the eggs inoculated as 18-day-old embryos hatched than of those inoculated at seven, 10 and 12 days old. The hatchability of the uninoculated village chicken eggs used in the four trials ranged from 60 to 100 per cent. Mycoplasmas were isolated from the oesophagus, trachea and yolk of the infected chicks and from the dead-in-shell embryos.