Mature seed samples from 35 populations of Echinochloa were collected in fi
elds of the Eastern Iberian Peninsula and evaluated for nine morphologic tr
aits. Four kernels per population were separately assayed for four isozyme
systems. Genetic variability components were studied by correspondence anal
ysis and chi-square distance using the isozyme pattern as variables. Geneti
c uniformity was found within populations as would be expected from their a
utogamous mating system. Intraspecific isozyme variability covered a wide s
pectrum, from none in E. colonum to very high in E. oryzoides. All the popu
lations of E. colonum clustered together based on morphometric and isozymat
ic characters, and also those of E. oryzicola, but were distinct from the p
opulations of all other species. Populations of E. crus-galli, E. hispidula
, and E. oryzoides clustered within species based on morphology, but did no
t cluster based on isozymatic characters, being mixed in the isozymatic den
drogram. Our results support the hypothesis that E. hispidula and E. oryzoi
des are infraspecific taxa of E. crus-galli. The large between-population g
enetic variability may explain the differential response to herbicides with
in a given species and indicate that weed specialists should study the diff
erences in response to a wide spectrum of Echinochloa populations to severa
l herbicides instead of testing many plants from a small number of populati
ons.