A replica technique was used in conjunction with transmission electron microscopy to determine the shape, size and degree of aggregation of fulvic and humic acid particles deposited on mica by the rapid freezing and drying of dilute aqueous solutions. Five major types of structures, common to both fulvic and humic acid preparations, were observed: small discrete spheroids (9-50 nm in diameter); flattened aggregates of spheroids (100-200 nm in diameter); linear, chainlike assemblies of these larger aggregates; flattened filaments (15-150 nm in width); and perforated sheetlike structures. Transitional stages were noted between these various structural conformations. The concentration of the humic substance in solution at the time of freezing and drying determined the type of structure that predominated in the deposits. Varying the pH (3.5, 7.0, 10.0) of the original solutions had no effect on the type of structures observed. The techniques introduced overcome many of the difficulties inherent in the preparation of humic substances for transmission electron microscopy and have potential use in the study of metal- and clay-complexing of these materials.