Studies of Portuguese maize (Zea mays L.) germplasm identified several varieties that had an abnormal ear shape. Six Portuguese Regional Varieties (PRVs) that had a high frequency of abnormal ear shape (fasciation expression) were selected for study. Qualitative and quantitative genetic analyses were conducted to determine the genetic systems involved and the inheritance of fasciation expression. Each of the six PRVs was crossed with three genetic stocks for ramosa expression (ra1, ra2, and ra3) to test for allelism between ramosa and fasciation expression. The quantitative genetic analyses included replicated field trials and a diallel series of crosses among eight sources to determine combining ability and S(,1) and S(,2) progenies of PRV 30 to determine the genetic variation and heritability of fasciation among progenies. Data were collected for 12 plant and ear traits. PRV 30 had the greatest frequency of fasciation expression and was studied in greatest detail;Analyses of crosses of PRVs with the three ramosa sources suggested there was no allelism between genes for fasciation and genes for ramosa. Fasciation generally showed a recessive behavior, but there were instances when partial dominance seemed to be present. Both ramosa 1 (ra1) and fasciation seemed to be associated with suppressor genes in their expression. For 12 traits related to fasciation expression, general combining effects were always greater than specific combining effects, indicating additive genetic effects were of greater importance. Fasciation was effective for increasing kernel-row number. There was evidence that fasciation was related to yield in an indirect manner by influencing ear length, which was highly correlated with yield. In crosses with the U.S. inbred A632, the negative influence of fasciation on ear length was prevented, and fasciation contributed positively to yield, probably because fasciation was highly correlated with kernel-row number;Both the qualitative and quantitative analyses showed that fasciation was inherited in a complex manner. Further research is needed to determine the relative importance of the trait in Portuguese germplasm and how it can be used most effectively. The abnormal ear trait may have potential to enhance productivity in crosses with other types of germplasm.