We have studied the formation process of pseudoisocyanine (PIC) J-aggregates by fluorescence detection of single aggregates in. owing solution. After the addition of sodium chloride as the initiator for aggregation, both continuous and pulse-like signals were observed. The continuous signal is the fluorescence of a large amount of 'meso-aggregates' consisting of 20-100 PIC molecules each. The pulsed signal is the fluorescence of individual 'macro-aggregates' of micrometer lengths. We discuss the formation process of these macro-aggregates on the basis of the temporal behavior observed. (C) 2008 Elsevier B.V. All rights reserved.