The most narrow classification a scientist can assign to a newly discovered organism is species.

Here's a breakdown of the taxonomic hierarchy, from broadest to most specific:

1. Domain: The broadest category, encompassing all of life (e.g., Bacteria, Archaea, Eukarya)

2. Kingdom: A major grouping within a domain (e.g., Animalia, Plantae, Fungi)

3. Phylum: A group of related classes (e.g., Chordata, Arthropoda)

4. Class: A group of related orders (e.g., Mammalia, Aves)

5. Order: A group of related families (e.g., Primates, Carnivora)

6. Family: A group of related genera (e.g., Hominidae, Felidae)

7. Genus: A group of closely related species (e.g., *Homo*, *Pan*)

8. Species: The most narrow classification, a group of organisms that can interbreed and produce fertile offspring.

While scientists can further refine a species designation using subspecies and varieties to distinguish populations with unique traits, species remains the most fundamental and narrow classification level.