Phospholipase C-gamma1 (PLC-gamma1) mediates cell adhesion and migration through an undefined mechanism. Here, we examine the role of PLC-gamma1 in cell-matrix adhesion in a hanging drop assay of cell aggregation. Plcg1 Null (-/-) mouse embryonic fibroblasts formed aggregates that were larger and significantly more resistant to dissociation than cells in which PLC-gamma1 is re-expressed (Null+ cells). Aggregate formation could be disrupted by inhibition of fibronectin interaction with integrins, indicating that fibronectin assembly may mediate aggregate formation. Fibronectin assembly was mediated by integrin alpha5beta1 in both cell lines, while assays measuring fibronectin assembly revealed increased assembly in the Null cells. Null and Null+ cells exhibited equivalent fibronectin mRNA levels and equivalent levels of fibronectin protein in pulse-labeling experiments. However, levels of secreted fibronectin in the conditioned medium were increased in Null cells. The data implicates a negative regulatory role for PLC-gamma1 in cell aggregation by controlling the secretion of fibronectin into the media and its assembly into fibrils.