V. O. Key’s proposition that one-party politics in the American states has been destructive of both the dominant and minor parties in such systems fits California’s political experience very nicely. From 1893 to 1932 California Republicans captured almost all statewide offices and they maintained huge legislative majorities. Furthermore, both parties entered the 1930’s with little institutionalized capacity for performing the basic nominating and campaigning functions. It appears, therefore, that the notorious weakness of political parties in California could be accounted for in terms of Key’s theories of the debilitating impact of one-party politics. Understandably, however, this line of speculation raises questions in the minds of those students of California politics who are aware of the several studies that particularly credit crossfiling for the weakness of California parties at mid-century. Consequently, the purpose of this article is to demonstrate the relationship between intensity of interparty competition and crossfiling in California politics during the first half of this century. In establishing this relationship, this study first shows that crossfiling did not become a general practice until after the end of California’s one-party era (1893-1932). The analysis then goes on to demonstrate that, during the one-party 1920’s, crossfiling occurred more often in the relatively competitive areas of the state than in the most profoundly one-party areas. Finally, a deviant case analysis develops the correspondence between the incidence of successful crossfiling (winning both parties’ primaries) and the relative competitiveness of types of offices.