Discount supermarket Lidl, which has 39 stores here, remains the fastest-growing grocer in Northern Ireland, with sales up 5.2% over the last year, according to a report.
But information company Kantar said Tesco remained the number one supermarket in Northern Ireland, and was the only one of the three main supermarkets to increase sales over the 12 months.
People in Northern Ireland made an average of 261 visits to supermarkets over the year - 10 more than during 2018, Kantar said.
And shoppers at Tesco were making an extra seven trips to its stores every year. Tesco, which has around 50 stores in Northern Ireland, increased its sales by 1.2%, and has a market share of 35.4%.
In contrast, Sainsbury's and Asda, which have 14 and 19 stores respectively, reported a fall in their sales growth, by 1.4% and 1.8% respectively.
Sainsbury's has a market share of 17%, giving it a slight lead on Asda, which has 16.6% of the market.
Charlotte Scott, consumer insight director at Kantar, said growth in the market was 0.7% in the year to December 1. She added: "Lidl was the best performing retailer this period with sales 5.2% higher than the previous year and it gained 0.3 percentage points of market share as a result... the retailer's performance was also helped by attracting new shoppers and encouraging bigger baskets."
Shoppers to Asda were making fewer trips and reducing the amount in their shopping baskets, with the latter trend also affecting Sainsbury's.
Ms Scott added: "Tesco remains the only one of the three largest retailers to achieve growth in the past 52 weeks - increasing its market share to 35.4% with sales up by 1.2%.
"An increase in shopper frequency outweighed falling basket sizes as Tesco customers made an average of seven more trips to the retailer this year than last year."