1. During artificial incubation of 8000 guinea fowl eggs, the effects of temperature, relative humidity, rate of inflowing air and age of the laying flocks were determined. 2. Total duration of incubation was divided into setter (0 to 24 days) and hatcher (25 to 28 days) periods. Eggs transferred at the end of the setter period, (ET, % of fertile eggs) and hatching rate (HR, % of transferred eggs) were calculated, as well as the total hatching rate expressed as percentage of fertile eggs. 3. In the range 36 to 39 degrees C, temperature affected significantly (P < 0.001) ET and HR, with optima at 37.2 and 37.0 degrees C +/- 0.1 degrees C respectively. 4. During incubating period, relative humidity in the range 40 to 64% (water vapour partial pressure, PH2O = 17 to 34 Torr at 36 to 39 degrees C), significantly (P < 0.001) affected the total diffusive water loss and hatchling mass, both expressed as percentage of fresh egg mass. ET was significantly affected by water loss, the highest ET being for water loss of 13.3 +/- 0.5%. Optimal relative humidity was calculated to be 48 to 52% (PH2O = 23 to 25 Torr at 37.2 degrees C). 5. The rate of inflowing air significantly (P < 0.001) affected HR, with an optimum at 3.1 LSTPD/(h.egg). 6. The age of the laying flock significantly (P < 0.001) increased water loss; this was explained by a parallel increase of the mass specific shell conductance to water vapour. 7. Finally optimum incubation conditions were deduced, giving total hatching rates of 78 to 81% of fertile eggs, improving by 5 to 8% the best results obtained routinely in commercial practice.