In 1951, the 600 was replaced by the 25 series, which like the 600 came in a four wheel (25R), tracked (25C) and tricycle version (25T), all with a number of variants for a total of nine types.
Later the 25 Calzolari with four wheel steering would join the series.
 
Visually the 25 was very similar to the 600, but had a new front axle design, smaller mudguards, and other minor details.
 
The petrol/diesel engine were the same as well, now slightly uprated to 23 hp at 1.750 rpm.

This combo-fuel engine would be replaced by two new engines in 1953. A diesel and a petrol engine. Though they ran on different fuels, they were basically the same engine, with respects to the different characteristics of the fuel they used. 
Both engines were 1.901 cc, water cooled, overhead valve engines, running at max 2.000 rpm, producing between 24 and 25 hp. And as a first for Fiat tractors, was now fitted with an electric 24 volt starter motor.

With the introduction of these new engines, Fiat abandoned the strong but expensive one"tub" chassis architecture, in favor of a more flexible setup, with the gearbox, differential and lift mechanism all part of one casing as before, - but the engine, radiator and engine cover now resting on two steel beams, connecting the rear casing to the front axle. This setup proved to be equally reliable to the tub.
And the undercarriage of the 601 crawler was modified to accommodate the new chassis, becoming the 25C.
But modifications didn't end with the 25C, as Fiat in 1953 introduced their first four wheel drive tractor. The 25R DT (Doppia Trazone).

The 25 Calzolari is a story of its own. It came to be in 1956, based on the Calzolari TC 27R4, developed by Officine Idro Meccaniche Fonderia Calzolari (Calzolari Hydro Mechanical Foundry Workshops), and was an evolution of Calzolaris first tractor, the TC15 from 1952.
The Calzolari TC 27R4 used Fiats petrol and diesel engines from the 25 series, and had four wheel steering that could be engaged or disengaged, locking differentials on both axles, drum brakes, and the option of reduction gear and a hydraulic lift. The perfect wheeled tractor for Italy's hills.
As an engine supplier, Fiat took notice of this machine, and convinced Calzolari to sell their tractor under the better known Fiat brand. Thus the Calzolari TC 27R4 became the Fiat 25 Calzolari.
 
The 25 series helped set a new record for Fiats tractor division in 1953.
10.000 tractors sold in a year, with the 25 series accounting for half the sales. The 25 staid the market leader in Italy till 1957, when it was overtaken by the Fiat "La Piccola".
 
In all, 26.000 diesel and 3.300 petrol were produced. Of that, 15.000 where wheeled tractors, and 14.000 crawler tractors.