Hapas, usually suspended in fertilized ponds, have long been used for tilapia fry production (Santiago et al., 1985; Bhujel, 1997) and recently for fry rearing. In fish breeding programs, members of a full-sib family usually share a common tank or hapa prior to tagging and communal testing. This tends to increase resemblance between family members which may reduce the efficiency of breeding programs. For convenience of monitoring and identification of families, hapas are often arranged in rows over the pond. If ponds are heterogeneous, for example with respect to nutrient availability, the spatial arrangement of hapas may create an environmental correlation among neighbouring units. The aim of this study was to quantify the common environmental and genetic effects on early rearing stages of tilapia in hapa-in-pond nursing conditions.