The study investigated the frequency of piglet diarrhoea from birth to weaning (33rd day) in a farrowing pigsty with 3 rows of boxes during 7 rearing cycles with 240 piglets each, always related to the day of birth in connection with the onset of the cycle. 3 groups were differentiated according to the day of birth: 3 and 4, 5, 6 and 7, based on the disposition of the sow. Also investigated was the position of the litters in the middle or one of the marginal rows of boxes of the pigsty. The germ density of the air during 2 cycles each of the rainy and dry seasons was investigated in the empty pigsty as well as after the 6th day of occupation by means of sedimentation in endo agar. A significant influence was found of the day birth (p less than 0.05) and of the interaction of row of boxes and day of birth (p less than 0.05) as well as the row of boxes and season (p less than 0.05) on the morbidity and mortality due to piglet diarrhoea. The higher levels occurred in the groups of animals born later and housed in the central row of boxes. The number of units forming colonies increased significantly (p less than 0.05) in 10 cm2/min samples on endo agar from the 0th to the 6th day of the cycle and reached 100% of the samples from which also E. coli K 88 was isolated.(ABSTRACT TRUNCATED AT 250 WORDS)