1. The development of the fleece and follicle population was studied in six Herdwick and two Herdwick × Swaledale sheep on their native farm in Cumberland. Six of these sheep survived the first 14 months of the experiment, and four were kept under observation for a further 12 months. Growth in body weight, body surface area, and area of tattooed squares was recorded and discussed.

2. Fibre type arrays (at the mid-side positions) were ‘Plateau’ in all but one of the pure Herdwick lambs, and ‘Saddle’ in the remaining three animals.

3. The earliest fibres of the array (halos and coarse super-sickles) were mostly ribbon-shaped in their pre-natal and early post-natal portions, below which they became cylindrical. These fibres were ‘birth coat kemps’ and were shed about 10 weeks after birth. At this time later-developed fibres (sickles and curly-tips) showed ‘crisis thinning’ but did not shed, whilst late curly tips and histerotrichs were not affected. These three groups gave rise to three corresponding groups of fibres in the adult fleece, namely kemps, hairs and fine fibres.

4. Two generations of kemp fibres were grown each year, ‘spring kemps’ commencing growth about April and shedding by July; and ‘autumn kemps’ commencing growth in July and forming ‘brush-roots’ in the autumn and early winter, although the majority remain in their follicles until the following spring.

5. The majority of the curly tip fibres of the birth coat grow into long hair fibres, which are of hetero-typical shape. In summer they are more or less coarse, and medullated, whilst in winter they become fine and lose their medulla; many cease growth during the middle of the winter period after producing a short length of fine fibre, and commence growth again in the spring. The late curly tip and histerotrichs fibres continue to grow as the fine undercoat fibres of the fleece, without any medullation.

6. A large proportion of both primary and secondary follicles shed their fibres during the winter months. When this proportion is sufficiently high this leads to a severe ‘winter break’ in the wool staple. The visible ‘break’ is the result of the combined effect of the winter fining of heterotype hairs and the shedding of some of all types. Fibres traversing the break were found to be either long heterotypes or long fine fibres.

7. The adult colour of any individual Herdwick sheep is the result of the interaction of several factors of which the main ones are: (1) later fibre types tend to be less pigmented than early types; (2) less vigorous growth of an individual fibre tends to reduce its pigmentation; (3) a more or less extensive acromelanic pigmentation; (4) the white areas may be affected by various degrees of speckling or roaning; (5) the kemp-like hairs of the face and legs are black at birth, and these are eventually shed and replaced by similar but unpigmented fibres.

8. Numerical data on the fibre and follicle population are presented. The mean proportion of the three main fibre types (all sheep, all seasons) was: coarse 18%, medium 25%, fine 59%. The mean adult follicle density was 1012 per sq.cm. and the mean adult S/P ratio was 3·37:1.