Study looked at men who had been exposed to pesticides
Dwindling sperm counts have been a cause of concern since Danish researchers reported in 1992 that sperm counts worldwide were declining.
Many experts blame environmental factors for the decline, as well as for an increase in testicular cancer.
The scientists studied 225 men from a productive farming region in Argentina. All of the men had attended an infertility clinic between 1995 and 1998.
They were quizzed about their lifestyle, medical history, occupation and exposure to pesticides. They also underwent a variety of physical and biological tests.
Although other factors may also be contributing to their infertility, the scientists said exposure to insecticides, herbicides and fungicides might have limited the men's ability to have children.
Low sperm levels
They found that exposure to pesticides was significantly associated with sperm levels well below the limit for male fertility.
The men who were exposed to pesticides also had higher levels of two female sex hormones in their system than men who had not come into contact with pesticides.
Dr Luc Multigner, of the French research institute INSERM, said the results suggested that the pesticides were having a toxic effect on the testicles and other sex glands.
He said: "The testicles are one of the most vulnerable organs to environmental, physical and chemical agents."
Argentina has become a major user of pesticides since the 1940s.
Dr Alejandro Oliva, from the Hospital Italiano Garibaldi in Rosario, Argentina, said: "There has been rising concern in many Western countries about the deleterious effects of environmental chemicals on male reproduction.
"Attention to this issue should now be given to developing countries."
The research is published in the journal Human Reproduction.