Mechanisms of partial unfolding and aggregation of proteins are of extreme interest in view of the fact that several human pathologies are characterized by the formation and deposition of protein-insoluble material, mainly composed of amyloid fibrils. Here we report on an experimental study on the heat-induced aggregation mechanisms, at basic pH, of concanavalin A (ConA), used as a model system. Thioflavin T (ThT) fluorescence and multiangle light scattering allowed us to detect different intertwined steps in the formation of ConA aggregates. In particular, the ThT fluorescence increase, observed in the first phase of aggregation, reveals the formation of intermolecular beta-sheet structure which constitutes a rate-limiting step of the process. The intertwining between the formation of beta-aggregate structures and the whole aggregation process is discussed as a function of protein concentration: a coagulation process produces the same kind of aggregates at the different concentrations studied. Multiangle light-scattering data highlight the onset of the condensation process which gives rise to formation of compact fractal aggregates. AFM microscopy supports this conclusion showing thin fibrils of ConA, formed in the early stage of aggregation, which further interact to form larger structures with a netlike spatial organization.