Nine sets of serial long-term density data for 4–24 species on two continents are examined for two types of synchronous fluctuation: congruent and incongruent. Comparing product/moment correlation coefficients for species pairs with corresponding values derived from matched controls is the basic method of synchrony detection. Congruent synchrony or fluctuation in unison is weakly to strongly expressed above the random level in all data sets. Evidence of incongruent synchrony is scant, but isolated examples of two subtypes are apparent. Congruent synchrony implies commonality of population dynamics among moderate numbers of species, and pervasive density-governing mechanisms.