Which level of taxonomic category indicates whether an organism is mammal or not?

The taxonomic category that indicates whether an organism is a mammal or not is Class.

Here's why:

* Kingdom: All animals belong to the Kingdom Animalia.

* Phylum: Mammals belong to the Phylum Chordata.

* Class: Mammals are specifically classified within the Class Mammalia.

* Order, Family, Genus, Species: These categories further classify mammals into specific groups.

So, if an organism is classified within the Class Mammalia, it is a mammal. If it's classified in any other Class, it's not a mammal.