The effect of ionic strength (I) on the formation of thermally induced aggregates by the 7S globular storage protein of soya, β-conglycinin, has been studied using atomic force microscopy. Aggregates were only apparent when I=<0.1, and had a fibrous appearance, with a height (diameter) of 8-11 nm. At high ionic strength (I=1.0) the aggregates appeared to associate into clumps. When aggregate formation was studied at I=0.2, it was clear that aggregation only began at temperatures above the main thermal transition for the protein at 75 o C, as determined by differential scanning calorimetry. This coincided with a small change in secondary structure, as indicated by circular dichroism spectroscopy, suggesting that a degree of unfolding was necessary for aggregation to proceed. Despite prolonged heating the size of the aggregates did not increase indefinitely, suggesting that certain β-conglycinin isoforms were able to act as chain terminators. At higher protein concentrations (1% w/v) the linear aggregates appeared to form large macroaggregates, which may be the precursors of protein gel formation. The ability of β-conglycinin to form such distinctive aggregates is discussed in relation to the presence of acidic inserts in certain of the β-conglycinin subunits, which may play an important role in limiting aggregate length.