Fleet registrations for the month compared to March last year increased by 2.7%, while business registrations to sub-25 fleets were up by 19.7%.
Registrations for the first quarter of the year, compared to the same period last year, are up by 4.2% for fleet and 18.2% for small businesses.
In terms of manufacturers, fortunes were mixed, with half of the top 10 best sellers having improved sales during March over the same month last year.
Those posting increases were Vauxhall, Ford, Renault, Toyota and Fiat, while those with falling fleet sales were Peugeot, Volkswagen, BMW, Nissan and Citroen.
Among the top ten, Nissan suffered the biggest fall in sales of more than 35% but only last month the company revealed that the lack of sales of the Primera in the fleet market was leading it to change its tactics and focus on its successes in the sports utility vehicle sector.
Nissan GB national fleet director Dave Murfitt said: 'Rather than pulling out of the fleet market, we are refocusing our efforts. I see little point in forcing product when demand falls. It will be more profitable for us to channel our efforts into the SUV niche sector.'
Citroen also recorded a fall but the company is reducing its unprofitable fleet business.
Seven of the cars in the best selling fleet models top 10 all increased their fleet sales. The remainder, Vauxhall's Astra and Vectra and the Ford Fiesta, all suffered falls.
The Ford Focus was again the most popular car with fleets, as it was with private buyers. Sales during March to fleets were up by more than 15% to 14,849 units, from 12,879 units in March 2003.
The Vauxhall Astra suffered the biggest fall in sales throughout the month, with company car drivers waiting for the new model, which is launched next month.
Renault's new Megane experienced the biggest rise in sales, increasing by more than 40% to 7,420 units.