Yersinia enterocolitica has two type III secretion systems (TTSS): The well characterized Ysc-Yop system and the relatively uncharacterized Ysa-Ysp system. Detection of Ysps in culture supernatants has only been observed in vitro when cultures are grown at low temperature (26 degrees C) and in high salt (290mM NaCl). Previous reports demonstrated that expression from the ysaE promoter was activated by high salt. In this study, we report a new environmental stimulus for ysa gene expression; in the presence of high salt, growth on solid surface stimulates expression 7-fold compared to growth in high salt broth. These new data indicate that, in the presence of salt, solid surface is an extremely robust signal for the Ysa system.