Brazil's primary aluminium output dropped by 5.5% year-on-year in September to 120,600 tonnes, said Abal, the Brazilian aluminium association.
The figure was also slightly below the 121,000 tonnes produced in August this year.
Companhia Brasileira do Aluminio (CBA), a unit of Votorantim Metais, increased its output to 35,400 tonnes in September, up from 33,700 tonnes in August after solving a technical issue that affected production for several months.
But production was still below its installed capacity of over 39,500 tpm. In September 2010, the company produced 39,100 tonnes.
Norsk Hydro's Albras was the largest producer in September, with 37,700 tonnes.
This was lower than the 38,900 tonnes produced in August, but higher than the 37,400 tonnes produced in September 2010.
Alcoa's output reached 29,200 tonnes, up from 29,100 tonnes last year.
BHP Billiton produced 1.4% more aluminium year-on-year, at 14,500 tonnes, while Novelis experienced a 50.6% drop on the year to 3,800 tonnes after perrmanently closing one of its two smelters in Brazil late in 2010.
Between January and September, Brazilian smelters produced 1.07 million tonnes, down by 6.9% from the 1.15 million tonnes churned out in the corresponding period last year.