On the discovery of area effects in morph frequency variation in the snail Cepaea nemoralis on the Marlborough Downs (Cain & Currey 1963 a) an intensive study with mark-release-recapture methods of a population subject to these effects was begun. The area investigated was divided into four quarters (downland grass) and two nettlepatches; the snail population is continuous through the area. Frequent visits were made during the snails' active season in 1962, 1963 and 1964; in 1965 and 1966 a single large sample was collected, marked and released in the spring. Five-banded shells are absent. The scoring of mid-bandeds with reduced bands being difficult, attention was concentrated on the principal colour morphs, yellow, pink and brown. Thrushes were predating the snails heavily in 1962, but the hard winter of 1962/3 removed them, and they have not yet returned in any numbers; the snail population does not seem to have been affected by this hard season. Throughout the period of investigation, the density of snails in the nettlepatches has been about ten times that in quarters 2, 3 and 4. Quarter 1 has less than half the density in the other quarters, and differs markedly from them and the nettlepatches in morph frequency, although it resembles quarter 2 strongly in vegetation. It is likely, therefore, that the snails are extremely localized in their wanderings, the more so as we had to collect from each division, take the samples away to be marked, and scatter them on release, each at random in its own division. Our estimates of migration from one division to another also suggest strong localization and perhaps homing. The total population in the area is about 3000 adults. Subpopulations have decreased in all divisions from 1963 to 1965. Adult survival rates are high, about 0.65 per year; no differential survival of the colour classes has been found, but much more data are needed. Predation by thrushes in 1962 was heavy in the summer, but not in proportion to snail density; the nettlepatches were proportionately much less predated, probably because of their dense growth of herbage. Winter predation, by contrast, was almost entirely in the nettlepatches, then showing expanses of bare earth. A check on our own efficiency of collecting shows that we are taking non-random samples in the quarters (downland grass) but not in the nettlepatches. We find pinks rather more conspicuous than yellows, but dark browns much less conspicuous than either. As the snails' bodies in this area are very dark, yellows appear green, pinks dirty straw-colour. It seems unlikely that browns are really more cryptic visually than yellows. Possibly their behaviour is different. The samples of predated shells taken by the thrushes agree very closely with our samples, and it appears that they also are predating with a bias (missing browns) in the quarters but not in the nettlepatches. Morph frequencies show no sign of alteration in the period of investigation, except that in 1963 and 1964 there was a drop in percentages of browns in quarters 2, 3 and 4, apparently not continuing in 1965 and 1966. (The compensatory increase is shared equally between pinks and yellows.) This could well be due to the sudden cessation of selection by thrushes, but the rate seems excessive if there was selection only on adults. There is no obvious relation between morph frequencies and population density. The growth in size of the juveniles in our samples has been studied through the snails' active season. It is clear that they take two years to reach maturity (i.e. to form the lip terminating the adult shell). This finding and the survival rates give a minimum generation interval of four years, and the actual period may be close to five in this area.