Dietetic chocolate, which includes all low calorie and sugarless chocolate candy, suffered a 33.8 per cent decline in US sales last year, according to ACNielson.
The decline in sales to $157 million is particularly surprising due to the category's success in recent years.
The figures from ACNielson show it was the US' fastest growing confectionery category.
The figures refer to all pre-packaged products sold in US food, drug and mass merchandiser stores combined. The figures excluding Wal-Mart.
In 2002 dietetic chocolate grew by 84.7 on the previous year to $64 million, followed by astonishing growth of 142.5 per cent in 2003 to $154 million.
Sales reached a high in 2004 of $237 million after further growth of 53.7 per cent.
The ACNielson figures also show that seasonal chocolate sales are another black hole for manufacturers.
The category suffered its fourth consecutive year of declining sales last year.
The category's sales, which in 2001 exceeded a billion dollars, decreased by a further 1.9 per cent to end 2005 at $920 million.
Previous declines in seasonal sales have been attributed to the blurring of special holiday lines with general chocolate sales.
Consumers at seasonal periods are becoming more likely to buy products that are available all year round. In the past they might have bought products that would only have been given at that time of the year, such as Christmas packaged chocolate.
The wider availability of seasonal lines, particularly products sold under private labels, and great competition in the chocolate industry in general, has contributed to greater price competition in the category.
Hershey and Masterfoods account for two-thirds of all US chocolate sales.
According to ACNielson total chocolate sales in the US for 2005 ended at $2.5 billion.