Modifying the nutritional constituents of a defined agar medium containing low levels of glucose, asparagine, and linoleic acid affected formation of synnemata and dark mycelial pigment by Ceratocystis ulmi. Except for a darkly pigmented variant, no pigment or synnemata were formed on the glucose and asparagine medium without linoleic acid. With the addition of linoleic acid, or the substitution of glucose with potassium acetate and/or asparagine with tyrosine, the medium was suitable for both dark pigment and synnemata formation by some C. ulmi isolates. Media without linoleic acid also supported pigment and synnema formation by some isolates when acetate and/or tyrosine were used as the carbon or nitrogen sources, respectively. Generally, C. ulmi isolates known to be more aggressive produced more synnemata and dark pigment on the media containing glucose and linoleic acid than did those isolates known to be less aggressive. Pathogenicity and cultural characteristics vary among isolates of Ceratocystis ulmi (Buism.) Moreau, the Dutch elm disease fungus (3, 5-7, 10-13, 19, 24), and an association has been shown to exist between pathogenicity and certain cultural characters (3, 6, 10, 11, 19). Synnema production on elm wood (19) and synnema and dark pigment formation on a defined synnema production medium (SPM) (10, 13) are associated with pathogenicity. More aggressive isolates generally produce more synnema on elm wood (19) and SPM (10, 13) and more pigment on SPM (13) than less aggressive ones. Since SPM is a defined medium, more in-depth studies on the nutritional factors affecting in vitro synnema and pigment production by C. ulmi now are possible. This report presents the results of studies conducted to determine the effects of varying the carbon, nitrogen, and/or linoleic