Summary.1May to September observations on a colony of c. 400Phoca vitulinain Shetland. The limiting factors of the habitat are described.2Daily 8 a.m. counts showed that numbers hauled out depended chiefly on swell or human disturbance (both erratic), to a lesser degree on the state of the tide and little, if at all, on rain and sun.3Relations with other species include: practically no overlap withHalichoerus grypus, indications that sea‐birds may be taken as food, and man as a long‐established enemy, mainly in the pupping season, when almost an entire generation may be wiped out.4In May and early June there was a great deal of play, apparently sexual, with “pairs” rolling together in the water. First pups appeared on June 14th when the play period ceased. The pupping season extended over three weeks.5Breeding behaviour proved largely aquatic. Pups may be born on tidal rocks or apparently even in the water. They found great difficulty in landing at first and spent most of their time at sea. Mothers guarded them closely for about three weeks and suckled them either in the shallows or ashore. Lactation lasted four weeks.6After the pups became independent, adults began to moult. Sexual play was not resumed and no coition was seen during this post‐pupping season.7Of the seals present at the beginning of the seasonc.15 per cent were yearlings and possibly only 70 per cent. were adult. The number of pups born was c. 18 per cent of the total population. Comparisons are made with a colony