The seasonal variations in diet, feeding chronology, condition and growth of a fishable population of plaice Pleuronectes platessa in two bays off the North Wales coast were investigated. The population, estimated to comprise 478,630 fish (~ 19%) of ages II - XV, occupied an area of 236.73 Km2 with an average biomass of 23.36 tonnes (0.11 g/m2). Data on diet and feeding chronology were obtained from fish collected by trawling at known intervals (usually every three hours) during a twenty· four hour period. The diet of 'small' (15.5 - 19.4 em) and 'medium' (19.5 - 20.4 em) fish was similar in each of the months for which data was collected 1. e. March, May, August, November and February. The two chief items were the polychaeta Pectinaria koren! and the lamellibranch Abra Alba. In addition the 'large' fish (30 - 35 em) also took Nereis spp. and the bivalve Tellina fabula in significant numbers. The diet varied seasonally in all size-groups; it was dominated by polychaetes in the cooler months and by mll'.lscs in the warmer months. Feeding was considerably sync~ronised amongst members of the population. Feeding chronology was deduced from variations in average stomach contents with time of day and from the occurrence of food in specific regions of the alimentary canal. In the cooler months - February, March and November - the following phases were recognised: Feeding Peak, Gastric Evacuation, Feeding Pause and Resumption of Feeding. These phases were obscured in May and August probably because higher temperatcres led to faster gastric evacuation; this in turn allowed larger meals and increased feeding frequencies, thereby reducing feeding synchrony in the population. Laboratory studies later showed that at a given temperature, the rate of gastric evac~ation increases with meal size (gm). (Rate (g/hr) • (meal size»). At all temperatures gastric evacuation time (GET) for a given meal depended on fish size (g). GET was propertional to (fiss size)0.42 for a meal expressed as % body weight and to (fish size)-0.068 - size tor a meal expressed as grams. When all other factors. were kept constant GET varied as (temperature)-1.03. An enpirical length-weight relationship was used to describe seasonal 2.946 condition factors (t-l = a L ). Peak condition occurred in June and July; condition was intermediate in autumn (deteriorating) and spring (improving) and poor in winter. The condition of the edge of otoliths was used to determine the limits of the growing season. Improvement in condition preceded seasonal growth in length which started in April and was virtually over by September. Growth was asymptotic in all age-groups and the function Y = A - B (e ) described the data adequately. Estimates of daily ration were made from field data on average stomach contents. The population apparently cropped 491.15 tonnes of benthic production annually. Energy budget calculations indicated that in energy terms 32. 33'ls of this intake was used for routine metabolism, 9.62% for growth ~~d 0.97% for reproduction. The remaining 57.08% was lost through egestion, conversion and excretion.