One hundred and eighty-five rural Bangladeshi children (80 boys and 105 girls) aged 4-27 months were observed to investigate developmental, behavioural, and environmental risk factors for diarrhoea during a 6-month period. Incidence of diarrhoea was found to be the highest among children aged 10-12 months. Children of this age group had the greatest exposure to environmental contaminants in the neighbourhood. Incidence of diarrhoea was the highest in hot, dry months. Risk factors for diarrhoea included: faecal contamination and garbage disposal in infant's outdoor play compound, crawling, contact of hand and mouth with contaminated materials, greater distance of household from water source, inadequate cleaning after defecation; dirt of child's face, presence of flies, feeding rotten food; insufficient washing of infant's and caretaker's hands before feeding rice meals or soft, wet foods; and lack of mothers' willingness to visit a modern (allopathic) health practitioner.