Cell-Cell adhesion and fusion is an essential component of many cellular and developmental processes. Few systems, however, allow for detailed cellular, genetic, genetic, and molecular genetic investigations of the molecule(s) responsible for cell fusion. The objective of this study is to investigate candidate cell-cell adhesion/fusion molecules. The model system studies is the adhesion and fusion of the specialized fusion organelles of gametes of the unicellular alga Chlamydomonas, three independent experimental approaches have been designed to study the adhesion/fusion molecules. (I) Use newly developed bioassays in combination with immunological methods to study proteins in isolated fertilization tubules that are involved in cell body adhesion/fusion. (II) Use fusion-defective insertional mt+ mutants to study adhesion/fusion. (III) Study the role of the mt+ gamete specific protein, FUSI, in adhesion and fusion.