1. A series of grazing behaviour observations on 10 sets of lactating identical twins is described. Four fortnights distributed over the main lactation period were covered. In three of the fortnights, the observations were made on six 24 hr. periods, while in the remaining fortnight the cows were observed on all 14 days.

2. Anoutstanding feature of the grazing behaviour of dairy cattle is its variability, due to external and internal conditions.

3. Of the external factors, climate (in a temperate zone) is relatively unimportant.

4. The quantity and quality of the herbage offered are of importance in modifying the grazing behaviour of dairy cattle. Under adverse pasture conditions there is, in general, an increase of total working time (grazing + ruminating): grazing time increases with scarcity of herbage and with mixed quality, while ruminating time is prolonged by poor quality grass.

5. Ruminating time is strongly dependent on the quantity and quality of the ingested grass, but the relationship is, for various reasons, obscured in a field trial such as the present.

6. The adaptation of grazing habits to new conditions is very rapid.

7. Feed requirement for milk production is the most important internal factor determining variation in the length of grazing time.

8. Inheritance × environment interactions as a source of variations in grazing times are due to the following factors:

(a) Differences amongst twin sets in lactational persistency, which presumably cause parallel differences in feed requirements.

(b) Differences in the response of high and low producing twin sets to varying pasture conditions.

(c) Non-analysable idiosyncrasies common to the members of some twin sets.

9. Twin sets graze and ruminate at a characteristic rate.