"The Fight Against Slavery" which Channel 20 will air in three consecutive two-hour segments tonight, Tuesday and Wednesday at 9 p.m. is not better than "Roots." It is simply different.

The series, a co-production of the BBC and Time-Life Television, was written by Evan Jones, a Jamaican who is the descendant of slaves and slaveowners. Jones in addition to having written the series, is the host for the series.

Unlike Alex Haley's efforts to trace his own familial history from West Africa to America. Jones concentrates on the fight against slavery in England and in Jamaica.

There are the now-familiar scenes on the slave ship - they are not quite as compelling as those we saw in the first and second episodes of "Roots" - the slave auction and life on the plantations in Jamaica, where the main crop was sugar cane rather than cotton.

Unlike "Roots," which concentrated on the continuing narrative of one slave and his descendants. "The Fight Against Slavery" uses the continuity of 84 years of one of the great crusades of history - the efforts by men such as William Wilberforce. William Pitt and Charles james Fox to abolish first the slave trade and, finally, slavery itself in all the British colonies.

In many ways, it is useful that "Roots" preceded the airing of "The Fight Against Slavery." "Roots" was a compelling personal drama. The BBC and Time-Life Television production, while it has some absorbing moments of drama that are based on personalities, relies in the main on historical fact to engage our attention.

If there is a central character in the evolution of this drama, it is William Wilberforce, who waged a battle against the slave trade initially and finally the institution of slavery, which was abolished by parliament two days before his death in the summer of 1833.

The scries is superbly acted and handsomely mounted. In addition to being compelling drama, it is also an excellent historical primer on a crucial period in the history of the institution of slavery and its effect on the New World outside the American colonies. It will not have the same dramatic effect on most of us that "Roots" had, but in its own way, it is just as absorbing.