The hypothesis that emigration of individuals in excess of resource carrying capacity acts as a population regulatory mechanism was tested experimentally using the desert pupfish (Cyprinodon macularius). When emigration was prevented, four pupfish populations monitored from May 1982 to March 1984 were unable to regulate numbers to resources. Numbers increased to a mean peak size 1.4 times greater than four pools open to emigration, followed by high mortality, a decline in body condition, reduced recruitment, and stunting. The pattern of overpopulation was similar to that observed in fenced populations of rodents. In contrast, pupfish in open pools had lower numbers, higher recruitment, better condition and growth, and higher total production. Emigration patterns were similar in all four open pools. Population size, rate of increase, and temperature affected emigration rates. Nearly twice as many males than females emigrated. Emigrant pupfish usually had poorer condition factors than residents. Pupfish showed a rapid and uniform increase in emigration when resources were reduced. Nearly one-half (42.2 and 41.8%) of pupfish populations emigrated from two open pools wherein resources were suddenly reduced by 50%. Many fewer fish emigrated from undisturbed control pools (15.2 and 16.0%). The results suggest that residency-emigratory behavior of pupfish can reliably and precisely effect changes in numbers to be in consonance with resources. They support emigration as sufficient to regulate pupfish numbers to resources in open systems without the need for other factors or mechanisms.