Spontaneously occurring coaggregation-defective mutants of Veillonella atypica PK1910 were isolated and used to study the interactions of veillonella with Streptococcus sanguis, Strep. gordonii and Strep. SM. Three classes of mutants were isolated. The first class failed to participate in lactose-inhibitable coaggregations with streptococcal cells. The second class of mutants failed to coaggregate by the lactose-non-inhibitable interactions. The third group, isolated from the first group, failed to coaggregate with any of the oral streptococci. Based on the coaggregation phenotypes of these mutants, a model for veillonella-streptococcal coaggregations is proposed, in which two distinct surface molecules mediate three types of interactions with the oral streptococci.